Crumpled Graphene Sheets Research Articles

Graphene-Wrapped Silicon Nanoparticles for All-Solid-State Lithium-Ion Batteries

Current lithium-ion batteries consist of a graphite anode and a metal oxide cathode. Due to their relatively high energy density and rechargeability, they have enabled various applications over the past few decades. However, significant improvements to battery cost, performance, and safety for applications such as vehicle electrification require a shift to next-generation materials, such as silicon as an anode which offers higher capacity, as well as solid-state electrolytes (SSEs) which are non-flammable. Both of these technologies currently suffer challenges which have prevented their widespread adoption. Silicon suffers low conductivity, and large volume change on each cycle, subsequently causing loss in electrical contact and the formation of an unstable solid electrolyte interface (SEI) each cycle. This project aims to solve these challenges by wrapping silicon nanoparticles and a sacrificial spacing material with crumpled graphene sheets using a scalable, spray drying method, and coupling it with a solid electrolyte. The graphene shell provides electrical contact with the silicon and space for its expansion during lithiation. The solid electrolyte provides a safer alternative to liquid electrolytes, and further reduces the SEI formation which, with a liquid electrolyte, can leak into such crumpled structures. In this work we will present our initial results concerning the engineering of void space within the shell’s core while simultaneously improving its conductivity to maximize the achievable capacity and cycle life. We will explore different spacing material candidates such as chitosan which, upon heat treatment of the sample, would not only provide space for the expansion of silicon, but the residual nitrogen-doped carbon would also act as a conductive filler inside the crumpled structure. We will also present our initial work involving the solid electrolyte synthesis including an investigation of the effect of solvents on the structure of the electrolyte as well as powder mixing and solution infiltration methods to introduce high conductivity SSEs and their performance when coupled to the graphene-wrapped silicon anode structures.

Sulfur modification of worm-like exfoliated graphite with crumpled graphene sheets derived from mesocarbon microbeads for electrochemical supercapacitors

Chemical exfoliation and unzipping of graphitic mesocarbon microbeads tend to form bone-like aggregates of graphene oxide (GO) sheets (marked as O-EMCMB). Heat treatment of O-EMCMB leads to the formation of partially reduced GO fragments (marked as HT-EMCMB) with hydrophobic sheets. Sulfur doping of O-EMCMB through hydrothermal process with sodium sulfide results in the formation of sulfur-doped and reduced GO with worm-like shape (marked as S-EMCMB). S-EMCMB has more crumpled sheets and hydrophilic surface than HT-EMCMB. Cyclic voltammetry reveals that S-EMCMB electrode gains more pseudocapacitive charge-storage capacity than the O-EMCMB and HT-EMCMB electrodes in 6 M KOH solution. S-EMCMB electrode exhibits high rate performance, cycling stability, and coulombic efficiency during galvanostatic charge and discharge (GCD) processes, its specific capacitance attains 314 F g−1, far greater than the O-EMCMB (67 F g−1) and HT-EMCMB (222 F g−1) under a GCD current of 1 A g−1. The enhanced charge-storage properties of S-EMCMB can be ascribed to its high hydrophilicity and electrical conductivity after incorporation of sulfur-related functional groups into the crumpled graphene sheets. This unique worm-like configuration inhibits graphene sheets from restacking and enables rapid transport of ions and electrons through hydrophilic pores for facilitating the redox kinetics at the electrolyte-electrode interfaces.

Flexoelectric electricity generation by crumpling graphene

We utilize atomistic simulations that account for point charges and dipoles to demonstrate that flexoelectricity, which arises from strain gradients, can be exploited to generate electricity from crumpled graphene sheets. Indentation of a circular graphene sheet generates localized developable (d)-cones, for which we verify the core radius and azimuthal angle with established theoretical models. We determine the voltage that can be generated based on the resulting electrostatic fields and compare the voltage generation to previous theoretical predictions that are scaled down to the nanoscale. In doing so, we find that the voltage generated from crumpling graphene exceeds, by about an order of magnitude, the expected voltage generation, indicating the benefit of exploiting the large strain gradients that are possible at the nanoscale. Finally, we demonstrate that crumpling may be a superior mechanism of flexoelectric energy generation as compared to bending of two-dimensional nanomaterials.

Thermally templated cobalt oxide nanobubbles on crumpled graphene sheets: A promising non-precious metal catalysts for acidic oxygen evolution

• Acid stable non-PGM Co oxide OER catalysts were synthesized via spray pyrolysis. • Morphology of Co oxides were tailored through thermal reduction and/or oxidation. • Co 3+ ion-rich hollow Co 3 O 4 spheres on rGO offered more active sites for the OER. • CG-RO showed remarkably higher stability than Ir black under potential cycling. Ru- or Ir-based precious metal oxides are typically applied as oxygen evolution reaction (OER) catalysts, owing to their high activity in acidic media and at high operating potentials. However, their expense and unsatisfactory durability prevent their application as anode catalysts in proton exchange membrane water electrolyzer stacks. This study presents the synthesis of various noble metal-free cobalt oxides supported on reduced graphene oxide catalysts using ultrasonic spray pyrolysis following different heat treatment processes. The catalyst prepared through additional thermal reduction and subsequent oxidation exhibits highly dispersed and Co 3+ ion-rich hollow Co 3 O 4 spheres on crumpled reduced graphene oxide sheets, thereby providing additional catalytically active sites for the OER. Under severe potential cycling, this catalyst exhibits acid-stability and a robust nanostructure, thereby demonstrating an extraordinarily superior stability in comparison to that of a commercially available Ir black catalyst.

Size-dependent structural behaviors of crumpled graphene sheets

Abstract Understanding the complex structural behaviors of crumpled graphene at a fundamental level is of critical importance in various engineering and technological applications. Here, we present a coarse-grained molecular dynamics (CG-MD) study for investigating structural behaviors of graphene sheets having varying sizes (or masses) during the crumpling process. The simulation results reveal that larger size graphene sheets at the initial two-dimension (2D) state tends to become more self-adhering and self-folding upon crumpling compared to the smaller ones, before forming a sphere-like highly crumpled structure. The fractal dimension of the crumpled graphene sheets, i.e., a measure of packing efficiency, is predicted to be D = 2.395 from our CG modeling, which is largely consistent with previous measurements. Remarkably, the size-dependence of various structural properties of the crumpled graphene sheets, including radius of gyration, hydrodynamic radius, and viscosity radius, can be quantitively described by the power-law scaling relationships during the crumpling process. By systematically evaluating shape descriptors, it is found that the overall crumpling behaviors of graphene sheets can be characterized by three different regimes (i.e., less, intermediate, and highly crumpled states), which are associated with edge-bending, self-adhesion, and further compression mechanisms, respectively. Our study provides fundamental insights into the size-dependent structural behavior of graphene sheets under crumpling, which is crucial to develop the structure-property relationships towards designing and engineering crumpled matter.

A facile synthesis of zeolitic analcime/spongy graphene nanocomposites as novel hybrid electrodes for symmetric supercapacitors

Abstract Zeolitic analcime/functionalized spongy graphene (FG/ANA) nanocomposites have been prepared using a facile hydrothermal-aided process. The electron microscopy imaging revealed the formation of 3-dimensional (3D) crumpled graphene sheets intercalated with analcime nanoneedles beneath the graphene layers. The intercalated ANA prevented the aggregation of FG sheets and allowed the diffusion of electrolyte ions into the FG layers. Upon electrochemical testing, the synthesized FG/ANA nanocomposite electrodes exhibited a maximum specific capacitance of 652 Fg−1 at 5 mV/s and 1061.95 Fg−1 at 0.5 A/g with 92% cycling stability at 1.5 A/g over 2000 cycles. The assembled symmetric supercapacitor device consisting of FG/ANA composite delivered a specific energy (E) and specific power (P) of 60.23 Wh/kg and 350 W/kg, respectively at 0.5 Ag−1. Therefore, the FG/ ANA composites demonstrate outstanding potential for use in constructing high-performance supercapacitors. The excellent capacitive performance can be ascribed to the improved wettability owing to the existence of oxygen and nitrogen as well as the spongy structure of the synthesized 3D graphene.

Graphene-Wrapped Silicon Nanoparticles for All-Solid-State Lithium-Ion Batteries

Current lithium-ion batteries consist of a graphite anode and a metal oxide cathode. Due to their relatively high energy density and rechargeability, they have enabled various applications over the past few decades. However, significant improvements to battery cost, performance, and safety for applications such as vehicle electrification require a shift to next-generation materials, such as silicon as an anode which offers higher capacity, as well as solid-state electrolytes (SSEs) which are non-flammable. Both of these technologies currently suffer challenges which have prevented their widespread adoption. Silicon suffers low conductivity, and large volume change on each cycle, subsequently causing loss in electrical contact and the formation of an unstable solid electrolyte interface (SEI) each cycle. This project aims to solve these challenges by wrapping silicon nanoparticles and a sacrificial spacing material with crumpled graphene sheets using a scalable, spray drying method, and coupling it with a solid electrolyte. The graphene shell provides electrical contact with the silicon and space for its expansion during lithiation. The solid electrolyte provides a safer alternative to liquid electrolytes, and further reduces the SEI formation which, with a liquid electrolyte, can leak into such crumpled structures. In this work we will present our initial results concerning how the spacing material affects the structure and how the overall structure can be modified by spray drying parameters to determine optimal conditions for capacity and cycle-life using, first, at typical, organic carbonate-based electrolyte. We will explore different spacing material candidates such as chitosan which, upon heat treatment of the sample, would not only provide space for the expansion of silicon, but the residual nitrogen-doped carbon would also act as a conductive filler inside the crumpled structure. Furthermore, we will investigate various binder systems and their impact on cycle-stability including polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), sodium alginate (SA), and cross-linked SA. Based on preliminary, both PAA and SA are shown to have improved performance compared to PVDF as well as higher cycle life, however, SA forms smoother, thinner electrode films. Cross-linked SA is also shown to increase the cycling life and decrease the capacity loss per cycle. We then explore powder mixing and solution infiltration methods to introduce high conductivity SSEs and their performance when coupled to the graphene-wrapped silicon anode structures.

Evaluation of antibacterial and cytotoxic properties of green synthesized Cu2O/Graphene nanosheets

Graphene nanocomposites have received attention for the therapy and detection of diseases. In this study, we developed a simple and green chemistry approach for synthesizing Cu2O/graphene nanocomposites (Cu2O/G) using date palm fruit syrup as a reducing agent. The graphene oxide surface anchored with Cu(OH)2 and reduced it to fabricate Cu2O-anchored graphene nanosheets using date palm fruit syrup. Physicochemical characteristics of the synthesized nanocomposites were analyzed. Scanning electron microscopy images revealed 50-70 nm Cu2O nanostructures anchored on the surface of crumpled graphene sheets. The Cu2O/G nanocomposites inhibited the gram-negative and gram-positive bacterial growth at 300 μg. When compared with Cu2O nanoparticles and graphene oxide nanosheets (GO), Cu2O/G nanocomposite exhibited outstanding bactericidal activity. The cytotoxic properties of the prepared nanocomposites were studied in human mesenchymal stem cells (hMSCs). The Cu2O/G nanocomposites did not reduced cell viability by up to 200 μg/mL and slightly induced cell death at high concentrations. However, Cu2O nanoparticles and GO have significantly reduced the cell viability in hMSCs. The microscopic images of cellular and nuclear morphology suggested that the Cu2O/G composites did not cause major changes to hMSCs. The Cu2O nanoparticles and GO remarkably triggers the cellular damages, nuclear condensation and DNA fragmentation in hMSCs. Our study results revealed that Cu2O/G has excellent antibacterial activity with good biocompatibility. Thus, Cu2O/G could be used as a promising antibacterial agent in various purposes.

Aerosol Synthesis of N and N-S Doped and Crumpled Graphene Nanostructures.

Chemically modified graphene–based materials (CMG) are currently attracting a vast interest in their application in different fields. In particular, heteroatom-doped graphenes have revealed great potentialities in the field of electrocatalysis as substitutes of fuel cell noble metal–based catalysts. In this work, we investigate an innovative process for doping graphene nanostructures. We optimize a novel synthetic route based on aerosol preparation, which allows the simultaneous doping, crumpling, and reduction of graphene oxide (GO). Starting from aqueous solutions containing GO and the dopant precursors, we synthesize N- and N,S-dual-doped 3D graphene nanostructures (N-cGO and N,S-cGO). In the aerosol process, every aerosol droplet can be considered as a microreactor where dopant precursors undergo thermal decomposition and react with the GO flakes. Simultaneously, thanks to the relatively high temperature, GO undergoes crumpling and partial reduction. Using a combination of spectroscopic and microscopic characterization techniques, we investigate the morphology of the obtained materials and the chemical nature of the dopants within the crumpled graphene sheets. This study highlights the versatility of the aerosol process for the design of new CMG materials with tailored electrocatalytic properties.

Toward the control of graphenic foams

Abstract Graphene-based materials are extensively studied, due to their excellent properties and their wide range of possible applications. Attention has recently been paid to three-dimensional-like graphenic structures, such as crumpled graphene sheets and graphenic foams: these kinds of materials can combine the properties of graphene associating high surface area and porosity, what is particularly interesting for energy or catalysis applications. Most of the synthesis methods leading to such structures are based on graphite oxide exfoliation and re-assembly, but in this work we focus on the preparation of graphenic foams by a solvothermal-based process. We performed a solvothermal reaction between ethanol and sodium at 220°C, during 72 h, under 200 bar, followed by a pyrolysis under nitrogen flow. An extended study of the influence of the temperature (800°C–900°C) of pyrolysis evidences an unexpected strong effect of this parameter on the characteristics of the materials. The optimal conditions provide multi-layer graphene (10 layers) foam with a surface area of 2000 m2·g−1. This work is an important step for the understanding of the mechanisms of the thermal treatment. Post-treatments in different experimental conditions are performed in order to modulate the structure and properties of the graphenic foams.

Facile synthesis of porous nitrogen-doped holey graphene as an efficient metal-free catalyst for the oxygen reduction reaction

Nitrogen-doped graphene is a promising candidate for the replacement of noble metal-based electrocatalysts for oxygen reduction reactions (ORRs). The addition of pores and holes into nitrogen-doped graphene enhances the ORR activity by introducing abundant exposed edges, accelerating mass transfer, and impeding aggregation of the graphene sheets. Herein, we present a straightforward but effective strategy for generating porous holey nitrogen-doped graphene (PHNG) via the pyrolysis of urea and magnesium acetate tetrahydrate. Due to the combined effects of the in situ generated gases and MgO nanoparticles, the synthesized PHNGs featured not only numerous out-of-plane pores among the crumpled graphene sheets, but also interpenetrated nanoscale (5–15 nm) holes in the assembled graphene. Moreover, the nitrogen doping configurations of PHNG were optimized by post-thermal treatments at different temperatures. It was found that the overall content of pyridinic and quaternary nitrogen positively correlates with the ORR activity; in particular, pyridinic nitrogen generates the most desirable characteristics for the ORR. This work reveals new routes for the synthesis of PHNG-based materials and elucidates the contributions of various nitrogen species to ORRs.

Liquid-solid-solution assembly of morphology-controllable Fe2O3/graphene nanostructures as high-performance LIB anodes

Abstract Fe 2 O 3 @rGO nanostructures with different Fe 2 O 3 morphologies (nanoplates and nanoparticles) have been successfully fabricated by a simple and novel liquid-solid-solution (LSS) hydrothermal method. The Fe 2 O 3 @rGO composites with different morphologies and sizes were obtained at the oil/water interface by simply adjusting the concentration of Fe 2+ ions and GO solution. Through the in situ solvothermal crystalization and growth process, Fe 2 O 3 can be uniformly anchored on the crumpled graphene sheets without aggregation. When acting as lithium ion battery anodes, the Fe 2 O 3 @rGO composite can deliver a reversible capacity of 1207 mA h g −1 at 0.2 A g −1 after 60 cycles, and a high rate capability of 548 mA h g −1 even under a high current density of 12.8 A g −1 (corresponds to a charge time of ~150 s). The high electrochemical performance indicates this synthesis technique may be a promising route to produce advanced anode materials for high-end lithium ion batteries.

Liquid-Solid-Solution Assembly of CoFe2O4/Graphene Nanocomposite as a High-Performance Lithium-Ion Battery Anode

Abstract CoFe 2 O 4 /graphene composites were fabricated via a novel one-pot liquid-solid-solution (LSS) hydrothermal process. Through ions electrostatic adsorption onto graphene sheets and oil microemulsion encapsulation, CoFe 2 O 4 nanoparticles can be uniformly anchored on crumpled graphene sheets without aggregation, and the size distribution of CoFe 2 O 4 particles can be controlled by the microemulsion shell in the range of 50–100 nm. With the synergistic effect between CoFe 2 O 4 and graphene, the CoFe 2 O 4 /graphene hybrid exhibits a high reversible specific capacity of 1102 mAh g −1 at 0.2 A g −1 after 100 cycles, and a good cycling stability as well. Moreover, the composite has good rate capability. The specific capacity can reach a high value of 410 mAh g −1 even under a high current density of 6.4 A g −1 (corresponds to a charge time of ∼230 s), indicating its promising application as an anode material for lithium ion batteries.

Simple fabrication of free-standing ZnO/graphene/carbon nanotube composite anode for lithium-ion batteries

Abstract A three-dimensional ZnO/graphene/CNT (carbon nanotube) composite was prepared by a one-step sol-gel synthetic technique followed by vacuum-assisted filtration. High resolution transmission electron microscopy revealed the formation of highly-dispersed ZnO nanoparticles, uniformly laid on the crumpled graphene sheets and CNT. The as-prepared ZnO/graphene/CNT composite exhibited excellent cyclability and rate capability when used as anode for lithium ion batteries, affording 620 mAh g −1 stable reversible discharge capacity after 100 cycles at 100 mA g −1 . The unique composite structure provided a large surface area and highly conductive network which maintains good electronic contact between particles, suppresses ZnO aggregation during the charge/discharge processes, and accommodates the large volume changes of ZnO upon cycling.

Enhanced rate capability of a lithium ion battery anode based on liquid–solid-solution assembly of Fe2O3 on crumpled graphene

We report a liquid–solid-solution assemble strategy to fabricate Fe2O3@graphene (Fe2O3@rGO) composites at the oil/water interface. The composite with ultrathin Fe2O3 nanoplates anchored on crumpled graphene sheets can act as a high-rate LIBs anode.

Nitrogen-doped and crumpled graphene sheets with improved supercapacitance

Nitrogen-doped thermally expanded graphene oxide (NtGO) was prepared by a facile thermal expansion and hydrothermal doping process. The thermal expansion process plays a vital role in improving the electrochemical performance of N-doped graphene by preventing its aggregation and improving its conductivity. The specific capacitance of NtGO is 270 F g−1 at a discharge current density of 1 A g−1 and the capacitance retention is 97% after 2000 cycles at this density. The strategy developed here provides an efficient and facile way to prepare nitrogen-doped graphene.

Synthesis of graphene based noble metal composites for glucose biosensor

Graphene (GR) based noble metal composites such as Pt–GR and Au–GR were synthesized for sensitive glucose biosensors. Aerosol spray pyrolysis (ASP) was employed to synthesize the noble metal nanoparticles–GR composites using a colloidal mixture of GO and noble metal precursor (H2PtCl6·6H2O and AuCl4·3H2O). The morphology of noble metal–GR composites was generally the shape of a crumpled paper ball, and the average composite size was about 1 µm. Crystalline Pt and Au nanoparticles less than 5 nm in diameter were uniformly deposited on the crumpled GR sheets, respectively. The charateristics of the as-prepared biosensors were tested through cyclic voltammetry measurements. All the biosensors based on the noble metal–crumpled GR composite exhibited a higher current flow as well as clear redox peaks, which resulted in a superior ability of the catalyst in terms of an electrochemical reaction. The amperometric response of the glucose biosensor based on the Pt–crumpled GR composite indicated the highest sensitivity as 62 µA/mM cm2.

Facile and rapid synthesis of highly crumpled graphene sheets as high-performance electrodes for supercapacitors

Highly crumpled graphene sheets (HCGSs) have been prepared by a facile and rapid route through freezing a chemically reduced graphene oxide aqueous suspension with liquid nitrogen. The porous and highly crumpled graphene structure with a large surface area and pore volume facilitates fast ionic transport within the electrode while preserving excellent electrical conductivity and thus endows HCGSs with excellent electrochemical properties. As the electrodes for supercapacitors, the HCGSs exhibit high specific capacitance (259 F g−1), excellent rate capability and cycling stability (93% retention after 5000 cycles).

Packing efficiency and accessible surface area of crumpled graphene

Graphene holds promise as an ultracapacitor due to its high specific surface area and intrinsic capacitance. To exploit both, a maximum surface area must be accessible while the two-dimensional (2D) graphene is deformed to fill volume. Here, we study stable crumpled graphene sheets of different lengths, $L$, using full atomistic molecular dynamics (MD) and determine a fractal dimension of $D\ensuremath{\cong}2.36\ifmmode\pm\else\textpm\fi{}0.12$, indicating efficient spatial packing. Introduction of defects inducing a transition from membrane-like to amorphous carbon further enhances packing efficiency. Further, variation of self-adhesion energy indicates a predominant role in randomly folded graphene. We determine that approximately 60% of the specific surface area of graphene is solvent accessible once crumpled and can be tuned with applied compression and crumpling. We analyze the solvent accessible surface area (SASA) and approximate the upper bound of free crumpled graphene capacitance to \ensuremath{\approx}329 F/g. Once crumpled, the achievable capacitance is highly dependent on the confined volume.

Intercalation of mesoporous carbon spheres between reduced graphene oxide sheets for preparing high-rate supercapacitor electrodes

A method for preparing three-dimensional (3D) carbon-based architectures consisting of mesoporous carbon spheres intercalated between graphene sheets is demonstrated in this paper. Colloidally dispersed negatively charged graphene oxide (GO) sheets strongly interacted with positively charged mesoporous silica spheres (MSS) to form a MSS–GO composite. The MSS were then used as template for replicating mesoporous carbon spheres (MCS) via a chemical vapor deposition process, during which the GO sheets were reduced to reduced graphene oxide (RGO). Removal of the silica spheres left behind a 3D hierarchical porous carbon architecture with slightly crumpled graphene sheets intercalated with MCS. The 3D carbon structure contained a low amount of oxygen (3.2% of atomic ratio of O/C) than a RGO sample (10.1%), which was prepared by using the chemical reduction method with hydrazine as the reducing agent. Thermal annealing of the 3D carbon structure in ammonia atmosphere further reduced the O/C atomic ratio to 1.6%. The capacitive performance of the samples as supercapacitor electrodes was investigated using the cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy techniques. The 3D carbon structure showed a substantially lower equivalent series resistance and a higher power capability than the RGO electrode. In addition, the 3D carbon electrode exhibited an excellent electrochemical cyclability with 94% capacitance retention after 1000 cycles of galvanostatic charge–discharge. The method demonstrated in this work opens up a new route to the preparation of 3D graphene-based architectures for supercapacitor applications.