The rich elemental composition, surface chemistry, and outstanding electrical conductivity of MXenes make them a promising class of two-dimensional (2D) materials for electrochemical energy storage. To translate these properties into high performance devices, it is essential to develop fabrication strategies that allow MXenes to be assembled into electrodes with tunable architectures and investigate the effect of their pore structure on the capacitive performance. Here, we report on the fabrication of MXene aerogels with highly ordered lamellar structures by unidirectional freeze-casting of additive-free Ti3C2T x aqueous suspensions. These structures can be subsequently processed into practical supercapacitor electrode films by pressing or calendering steps. This versatile processing route allows a wide control of film thickness, spacing within lamellae (to give electrolyte accessible sites), and densities (over 2 orders of magnitude) and hence gives control over the final properties. The as-prepared MXene aerogel with a density of 13 mg cm–3 achieves 380 F g–1 capacitance at 2 mV s–1 and 75 F g–1 at 50 mV s–1. The calendering of the MXene aerogel into a porous 60 μm thick film with a density of 434 mg cm–3 leads to a superior rate capability of 309 F g–1 at 50 mV s–1. In addition, the rolled electrodes present an improvement in volumetric capacitance of 104 times as compared to the as-prepared MXene aerogel. Finally, the outstanding cyclability of rolled electrodes strengthens their nomination for supercapacitor applications. In this paper we demonstrate the possibilities in tuning the porosity and the electrochemical properties of aerogels highlighting the importance of evaluating new and hybrid processing methods to develop energy storage applications. The simplicity and versatility of the developed fabrication strategy open opportunities for the utilization of MXene lamellae architectures in a wide range of applications requiring controlled porosity including catalysis, filtration, and water purification.

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2D transition metal carbides and nitrides (MXenes) have shown outstanding potential as electrode materials for energy storage applications due to a combination of metallic conductivity, wide interlayer spacing, and redox-active, metal oxide-like surfaces capable of exhibiting pseudocapacitive behavior. It is well known that 2D materials have a strong tendency to restack and aggregate, due to their strong van der Waals interactions, reducing their surface availability and inhibiting electrochemical performance. In order to overcome these problems, work has been done to assemble 2D materials into 3D porous macrostructures. Structuring 2D materials in 3D can prevent agglomeration, increase specific surface area and improve ion diffusion, whilst also adding chemical and mechanical stability. Although still in its infancy, a number of papers already show the potential of 3D MXene architectures for energy storage, but the impact of the processing parameters on the microstructure of the materials, and the influence this has on electrochemical properties is still yet to be fully quantified. In some situations the reproducibility of works is hindered by an oversight of parameters which can, directly or indirectly, influence the final architecture and its properties. This review compiles publications from 2011 up to 2020 about the research developments in 3D porous macrostructures using MXenes as building blocks, and assesses their application as battery and supercapacitor electrodes. Recommendations are also made for future works to achieve a better understanding and progress in the field. carbon and/or nitrogen and T x represents surface terminations which vary depending on the synthesis method used (for example, hydroxyl, oxygen, or fluorine) [10].With less than 10 years since their discovery, efforts are still mainly pointed towards the assessment of its potentialities and exploration of its properties, while their integration and application in various engineering fields are still very much in their infancy. Nevertheless, investigations have shown very competitive results in energy storage devices attracting a lot of interest in the field. In particular, the inner conductive metal carbide layers provide fast electron supply to electrochemically active sites, and functional groups on the surface, allow water intercalation and fast redox activity for pseudocapacitive energy storage [11][12][13]. Besides energy storage, MXenes also showed promising properties and have been researched for a variety of other areas, such as electromagnetic shielding, environmental, sensors, optoeletronic devices, and renewable energy [9,[14][15][16].For most practical applications, the MXene powders must be assembled or processed into a macroscopic sample such as, for example, an electrode. Conventionally this is done either by mixing it with a solvent to form a slurry which is painted over a substrate, by directly vacuum filtration, by spin coating, or by spraying the MXene suspension to form a compact film. During these processes, there i...

Here, we report a novel electrode structure with graphene and graphene-carbon black hybrids by electrospraying for polymer electrolyte membrane fuel cells. After syntheses of platinum (Pt)/partially reduced graphene oxide (rGO) and Pt/r-GO/carbon black (CB) hybrid electrocatalysts, suspensions of synthesized electrocatalyst inks were prepared with Nafion Ò ionomer and poly(vinylidene fluoride-cohexafluoropropylene) and electrosprayed over carbon paper to form electrodes. Electrosprayed catalyst layer exhibited uniform and small size Pt distribution. As the graphene content increases micrometer-sized droplet, pore formation and surface roughness of the electrode increase. Thus, an open porous electrode structure which is favorable for mass transport is achieved by electrospraying. The maximum power densities, 324 mW cm -2 for Pt/rGO and 441 mW cm -2 for Pt/rGO/CB electrosprayed electrodes, were achieved at a relatively low catalyst loading.