Tailoring the Microstructure of Lamellar Ti3C2Tx MXene Aerogel by Compressive Straining.

Aerogels are attracting increasing interest due to their functional properties, such as lightweight and high porosity, which make them promising materials for energy storage and advanced composites. Compressive deformation allows the nano- and microstructure of lamellar freeze-cast aerogels to be tailored toward the aforementioned applications, where a 3D nanostructure of closely spaced, aligned sheets is desired. Quantitatively characterizing their microstructural evolution during compression is needed to allow optimization of manufacturing, understand in-service structural changes, and determine how aerogel structure relates to functional properties. Herein we have developed methods to quantitatively analyze lamellar aerogel domains, sheet spacing, and sheet orientation in 3D and to track their evolution as a function of increasing compression through synchrotron phase contrast X-ray microcomputed tomography (µCT). The as-cast domains are predominantly aligned with the freezing direction with random orientation in the orthogonal plane. Generally the sheets rotate toward flat and their spacing narrows progressively with increasing compression with negligible lateral strain (zero Poisson's ratio). This is with the exception of sheets close to parallel with the loading direction (Z), which maintain their orientation and sheet spacing until ∼60% compression, beyond which they exhibit buckling. These data suggest that a single-domain, fully aligned as-cast aerogel is not necessary to produce a post-compression aligned lamellar structure and indicate how the spacing can be tailored as a function of compressive strain. The analysis methods presented herein are applicable to optimizing freeze-casting process and quantifying lamellar microdomain structures generally.

Ice-templated hybrid graphene oxide-graphene nanoplatelet lamellar architectures: tuning mechanical and electrical properties.

The traditional freeze-casting route for processing graphene-based aerogels is generally restricted to aqueously dispersed flakes of graphene oxide (GO) and post-processing reduction treatments, which brings restrictions to the aerogels electrical properties. In this work, we report a versatile aqueous processing route that uses the ability of GO todisperse graphene nanoplatelets (GNP) to produce rGO-GNP lamellar aerogels via unidirectional freeze-casting. In order to optimise the properties of the aerogel, GO-GNP dispersions were partially reduced by L-ascorbic acid prior to freeze-casting to tune the carbon and oxygen (C/O) ratio. The aerogels were then heat treated after casting to fully reduce the GO. The chemical reduction time was found to control the microstructure of the resulting aeorgels and thus to tune their electrical and mechanical properties. An rGO-GNP lamellar aerogel with density of 20.8 ± 0.8 mg cm-3 reducing using a reduction of 60 min achieved an electrical conductivity of 42.3 S m-1. On the other hand, an optimal reduction time of 35 min led to an aerogel with compressive modulus of 0.51 ±0.06 MPa at a density of 23.2 ± 0.7 mg cm-3, revealing a compromise between the tuning of electrical and mechanical properties. We show the present processing route can also be easily applied to produce lamellar aerogels on other graphene-based materials such as electrochemically exfoliated graphene.

Alkali-cation-incorporated and functionalized iron oxide nanoparticles for methyl blue removal/decomposition.

Enhancing the rate of decomposition or removal of organic dye by designing novel nanostructures is a subject of intensive research aimed at improving waste-water treatment in the textile and pharmaceutical industries. Despite radical progress in this challenging area using iron-based nanostructures, enhancing stability and dye adsorption performance is highly desirable. In the present manuscript alkali cations are incorporated into iron oxide nanoparticles (IONPs) to tailor their structural and magnetic properties and to magnify methyl blue (MB) removal/decomposition capability. The process automatically functionalizes the IONPs without any additional steps. The plausible mechanisms proposed for IONPs incubated in alkali chloride and hydroxide solutions are based on structural investigation and correlated with the removal/adsorption capabilities. The MB adsorption kinetics of the incubated IONPs is elucidated by the pseudo second-order reaction model. Not only are the functional groups of -OH and -Cl attached to the surface of the NPs, the present investigation also reveals that the presence of alkali cations significantly influences the MB adsorption kinetics and correlates with the cation content and atomic polarizability.

Unravelling the Mechanism of Rechargeable Aqueous Zn-MnO2 Batteries: Implementation of Charging Process by Electrodeposition of MnO2.

Poor cycling stability and mechanistic controversies have hindered the wider application of rechargeable aqueous Zn-MnO2 batteries. Herein, direct evidence was provided of the importance of Mn2+ in this type of battery by using a bespoke cell. Without pre-addition of Mn2+ , the cell exhibited an abnormal discharge-charge profile, meaning it functioned as a primary battery. By adjusting the Mn2+ content in the electrolyte, the cell recovered its charging ability through electrodeposition of MnO2 . Additionally, a dynamic pH variation was observed during the discharge-charge process, with a precipitation of Zn4 (OH)6 (SO4 )⋅5H2 O buffering the pH of the electrolyte. Contrary to the conventional Zn2+ intercalation mechanism, MnO2 was first converted into MnOOH, which reverted to MnO2 through disproportionation, resulting in the dissolution of Mn2+ . The charging process occurred by the electrodeposition of MnO2 , thus improving the reversibility through the availability of Mn2+ ions in the solution.

Direct 3D printing of graphene using capillary suspensions.

Conventional 3D printing of graphene requires either a complex formulation of the ink with large quantities of polymers or essential post-processing steps such as freeze drying to allow printability. Here we present a graphene capillary suspension (GCS) containing 16.67 wt% graphene nanoparticles in aqueous suspension with 3.97 wt% carboxymethyl cellulose (CMC) as a stabiliser and a small quantity of the immiscible liquid octanol. This is shown to have the appropriate rheological properties for 3D printing, which is demonstrated through the fabrication of a simple lattice structure by direct writing and air drying at room temperature. The printed structure has a porosity of 81%, is robust for handling with a compression strength of 1.3 MPa and has an electrical conductivity of 250 S m-1. After heat treatment at 350 °C conductivity is 2370 S m-1 but the strength reduces to 0.4 MPa. X-Ray tomography of the internal architecture after printing shows the formation of the capillary suspension eliminates ordering of the 2D materials during extrusion through the printer nozzle. Thus capillary suspensions can be used to direct write graphene 3D structures without the necessity of complicated drying steps or burn-out of large quantities of polymer additions, facilitating shape retention and property control as compared to current 2D material ink formulations used for 3D printing.

3D Printing of Freestanding MXene Architectures for Current-Collector-Free Supercapacitors.

Additive manufacturing (AM) technologies appear as a paradigm for scalable manufacture of electrochemical energy storage (EES) devices, where complex 3D architectures are typically required but are hard to achieve using conventional techniques. The combination of these technologies and innovative material formulations that maximize surface area accessibility and ion transport within electrodes while minimizing space are of growing interest. Herein, aqueous inks composed of atomically thin (1-3 nm) 2D Ti3 C2 Tx with large lateral size of about 8 µm possessing ideal viscoelastic properties are formulated for extrusion-based 3D printing of freestanding, high specific surface area architectures to determine the viability of manufacturing energy storage devices. The 3D-printed device achieves a high areal capacitance of 2.1 F cm-2 at 1.7 mA cm-2 and a gravimetric capacitance of 242.5 F g-1 at 0.2 A g-1 with a retention of above 90% capacitance for 10 000 cycles. It also exhibits a high energy density of 0.0244 mWh cm-2 and a power density of 0.64 mW cm-2 at 4.3 mA cm-2 . It is anticipated that the sustainable printing and design approach developed in this work can be applied to fabricate high-performance bespoke multiscale and multidimensional architectures of functional and structural materials for integrated devices in various applications.

Understanding Mechanical Response of Elastomeric Graphene Networks.

Ultra-light porous networks based on nano-carbon materials (such as graphene or carbon nanotubes) have attracted increasing interest owing to their applications in wide fields from bioengineering to electrochemical devices. However, it is often difficult to translate the properties of nanomaterials to bulk three-dimensional networks with a control of their mechanical properties. In this work, we constructed elastomeric graphene porous networks with well-defined structures by freeze casting and thermal reduction, and investigated systematically the effect of key microstructural features. The porous networks made of large reduced graphene oxide flakes (>20 µm) are superelastic and exhibit high energy absorption, showing much enhanced mechanical properties than those with small flakes (<2 µm). A better restoration of the graphitic nature also has a considerable effect. In comparison, microstructural differences, such as the foam architecture or the cell size have smaller or negligible effect on the mechanical response. The recoverability and energy adsorption depend on density with the latter exhibiting a minimum due to the interplay between wall fracture and friction during deformation. These findings suggest that an improvement in the mechanical properties of porous graphene networks significantly depend on the engineering of the graphene flake that controls the property of the cell walls.

Self-healing graphene-based composites with sensing capabilities.

A self-healing composite is fabricated by confining a supramolecular polymer in a graphene network. The network provides electrical conductivity. Upon damage, the polymer is released and flows to reform the material. Healing is repeatable and autonomous. The composite is sensitive to pressure and flexion and recovers its mechanical and electrical properties even when rejoining cut surfaces after long exposure times.

Printing in three dimensions with graphene.

Responsive graphene oxide sheets form non-covalent networks with optimum rheological properties for 3D printing. These networks have shear thinning behavior and sufficiently high elastic shear modulus (G') to build self-supporting 3D structures by direct write assembly. Drying and thermal reduction leads to ultra-light graphene-only structures with restored conductivity and elastomeric behavior.

Mesoscale assembly of chemically modified graphene into complex cellular networks.

The widespread technological introduction of graphene beyond electronics rests on our ability to assemble this two-dimensional building block into three-dimensional structures for practical devices. To achieve this goal we need fabrication approaches that are able to provide an accurate control of chemistry and architecture from nano to macroscopic levels. Here, we describe a versatile technique to build ultralight (density ≥1 mg cm(-3)) cellular networks based on the use of soft templates and the controlled segregation of chemically modified graphene to liquid interfaces. These novel structures can be tuned for excellent conductivity; versatile mechanical response (elastic-brittle to elastomeric, reversible deformation, high energy absorption) and organic absorption capabilities (above 600 g per gram of material). The approach can be used to uncover the basic principles that will guide the design of practical devices that by combining unique mechanical and functional performance will generate new technological opportunities.

Designing smart particles for the assembly of complex macroscopic structures.

Particle get-together: Surface functionalization with a branched copolymer surfactant is used to create responsive inorganic particles that can self-assemble in complex structures. The assembly process is triggered by a pH switch that reversibly activates multiple hydrogen bonds between ceramic particles (see picture; yellow) and soft templates (n-decane; green).