The current quest for two-dimensional transition metal carbides and nitrides (MXenes) has been to circumvent the slow, hazardous, and laborious multistep synthesis procedures associated with conventional chemical MAX phase exfoliation. Here, we demonstrate a one-step synthesis method with local Ti₃AlC₂ MAX to Ti₃C₂T_z MXene conversion on the order of milliseconds, facilitated by proton production through solution dissociation under megahertz frequency acoustic excitation. These protons combined with fluorine ions from LiF to selectively etch the MAX phase into MXene, whose delamination is aided by the acoustic forcing. These results have important implications for the future applicability of MXenes, which crucially depend on the development of more efficient synthesis procedures. For proof-of-concept, we show that flexible electrodes fabricated by this method exhibit comparable electrochemical performance to that previously reported.

A novel strategy utilizing high‐frequency (10 MHz) hybrid sound waves to dramatically enhance hydrogen evolution reactions (HER) in notoriously difficult neutral electrolytes by modifying their network coordination state is presented. Herein, the practical limitations associated with existing electrolyzer technology is addressed, including the need for highly corrosive electrolytes and expensive electrocatalysts, by redefining conceptually‐poor hydrogen electrocatalysts in neutral electrolytes. The improvement in HER performance is attributed to the unique capability of the intense local electromechanical coupling arising from the acoustic‐forcing to ‘frustrate’ the tetrahedrally‐coordinated hydrogen bond network of water molecules at the electrode–electrolyte interface, resulting in the generation of a high concentration of “free” water molecules that are more readily able to access catalytic sites on the unmodified polycrystalline electrode. Together with the other synergistic effects that accompany the acoustic excitation (e.g., hydronium ion generation, convective relaxation of diffusion mass transfer limitations, and prevention of bubble build‐up and their removal from the electrode), the resultant overpotential reduction of 1.4 V at −100 mA cm⁻² and corresponding 14‐fold increase in current density, together with a net‐positive energy saving of 27.3%, showcases the potential of the technology as a scalable platform for effectively enhancing the efficiency of green hydrogen production.

Two-dimensional (2D) graphitic materials are often incorporated with metal–organic frameworks (MOFs) to compensate for the poor electrical conductivity of the latter, particularly if they are to be used for electrochemical applications. Nevertheless, the incompatibility between conventional three-dimensional (3D) MOF crystal morphologies with the 2D graphitic sheets often poses a challenge in the fabrication of planar heterostructures, therefore necessitating the addition of polymeric supports and binders. Herein, we report an acoustomicrofluidic method that enables rapid—within milliseconds—synthesis of homogeneous MOF/graphene oxide (GO) planar heterostructures without the need for such additives. In particular, the method harnesses the combination of the intense shear flow and slender-body geometrical confinement imposed by the spreading of a thin liquid film under high frequency electromechanical surface vibrations to drive in situ crystallization and in-plane templating of quasi-2D MOF crystals onto GO sheets simultaneously aligned into stacked layers by the flow. Unlike the 3D morphology of its conventional copper (Cu)-based MOF equivalent (HKUST-1), the unique quasi-2D MOFs that are produced possess large surface-area-to-volume-ratios and dense exposure of Cu sites on their surface that facilitate effective Cu/GO coordination. This ability to template the MOF directly onto the GO sheets allows for the construction of freestanding membranes that are sufficiently robust to withstand reduction of the GO to reduced graphene oxide (rGO) without compromising the integrity of the MOF crystal structure. We show that the synergistic coupling between the conductive rGO layers together with the redox-active surface Cu sites of the quasi-2D MOF enables excellent supercapacitor performance of this hybrid material—up to 292 F g⁻¹ at a current density of 1 A g⁻¹, which is approximately two orders of magnitude larger than its 3D HKUST-1/rGO counterpart.