The scalable and sustainable manufacture of thick electrode films with high energy and power densities is critical for the large-scale storage of electrochemical energy for application in transportation and stationary electric grids. Two-dimensional nanomaterials have become the predominant choice of electrode material in the pursuit of high energy and power densities owing to their large surface-area-to-volume ratios and lack of solid-state diffusion. However, traditional electrode fabrication methods often lead to restacking of two-dimensional nanomaterials, which limits ion transport in thick films and results in systems in which the electrochemical performance is highly dependent on the thickness of the film. Strategies for facilitating ion transport-such as increasing the interlayer spacing by intercalation or introducing film porosity by designing nanoarchitectures-result in materials with low volumetric energy storage as well as complex and lengthy ion transport paths that impede performance at high charge-discharge rates. Vertical alignment of two-dimensional flakes enables directional ion transport that can lead to thickness-independent electrochemical performances in thick films. However, so far only limited success has been reported, and the mitigation of performance losses remains a major challenge when working with films of two-dimensional nanomaterials with thicknesses that are near to or exceed the industrial standard of 100 micrometres. Here we demonstrate electrochemical energy storage that is independent of film thickness for vertically aligned two-dimensional titanium carbide (TiCT ), a material from the MXene family (two-dimensional carbides and nitrides of transition metals (M), where X stands for carbon or nitrogen). The vertical alignment was achieved by mechanical shearing of a discotic lamellar liquid-crystal phase of TiCT . The resulting electrode films show excellent performance that is nearly independent of film thickness up to 200 micrometres, which makes them highly attractive for energy storage applications. Furthermore, the self-assembly approach presented here is scalable and can be extended to other systems that involve directional transport, such as catalysis and filtration.

Programmable shape-shifting materials can take different physical forms to achieve multifunctionality in a dynamic and controllable manner. Although morphing a shape from 2D to 3D via programmed inhomogeneous local deformations has been demonstrated in various ways, the inverse problem-finding how to program a sheet in order for it to take an arbitrary desired 3D shape-is much harder yet critical to realize specific functions. Here, we address this inverse problem in thin liquid crystal elastomer (LCE) sheets, where the shape is preprogrammed by precise and local control of the molecular orientation of the liquid crystal monomers. We show how blueprints for arbitrary surface geometries can be generated using approximate numerical methods and how local extrinsic curvatures can be generated to assist in properly converting these geometries into shapes. Backed by faithfully alignable and rapidly lockable LCE chemistry, we precisely embed our designs in LCE sheets using advanced top-down microfabrication techniques. We thus successfully produce flat sheets that, upon thermal activation, take an arbitrary desired shape, such as a face. The general design principles presented here for creating an arbitrary 3D shape will allow for exploration of unmet needs in flexible electronics, metamaterials, aerospace and medical devices, and more.

Two-dimensional liquid-crystal elastomer (LCE) sheets with preprogrammed topological defects are prepared by aligning liquid-crystal monomers within micropatterned epoxy channels, followed by photopolymerization. Upon heating, the LCE films form various three-dimensional structures in agreement with theoretical design. The miniaturized LCE actuators offer large-area work capacities (≈1.05 J m ) to lift over 700 times their own weight.

Macroscale robotic systems have demonstrated great capabilities of high speed, precise, and agile functions. However, the ability of soft robots to perform complex tasks, especially in centimeter and millimeter scale, remains limited due to the unavailability of fast, energy-efficient soft actuators that can programmably change shape. Here, we combine desirable characteristics from two distinct active materials: fast and efficient actuation from dielectric elastomers and facile shape programmability from liquid crystal elastomers into a single shape changing electrical actuator. Uniaxially aligned monoliths achieve strain rates over 120%/s with energy conversion efficiency of 20% while moving loads over 700 times the actuator weight. The combined actuator technology offers unprecedented opportunities towards miniaturization with pre-1 arXiv:1904.09606v1 [cond-mat.soft] 21 Apr 2019 cision, efficiency, and more degrees of freedom for applications in soft robotics and beyond.Force generation, efficiency, strength-to-weight ratio, work capacity, and shape programmability will be key for the next generation of soft robots to perform complex functions. Despite significant advances in robots, including gymnastic feats (1), the underlying rigid actuation mechanisms, use of electric motors or hydraulic and pneumatic actuators, remain relatively unchanged and potentially hinder their miniaturization and, more importantly, their use in human collaborative environments (2). Efficient and programmable soft actuators, like an artificial muscle, would significantly increase the capabilities and potential applications of soft robotic systems in aerospace, industrial, or medical technologies (3-5). Among many soft actuation mechanisms that have been explored, dielectric elastomer (DE) actuators appear promising and even outperform skeletal muscle in some aspects (6-9). Separately, liquid crystal elastomers (LCEs) have demonstrated reversible large mechanical deformation by thermal and optical actuation. Recent advances in photoalignment and top-down microfabrication techniques have enabled pre-programming of LC alignment in microdomains for complex shape morphing (10-12). However, both actuator types have their drawbacks: DE films need to be prestretched, making it difficult to program local actuation behaviors microscopically (13). Meanwhile, directly converting electrical energy to mechanical work utilizing LCEs has, until now, remained limited due to the small strain generated (14-19). Typically, DE actuators function by electrostatic attraction between two compliant electrodes coated on opposing sides of an isotropic DE to form a variable resistor-capacitor (20).High voltage applied to the compliant electrodes induces an electrostatic pressure called Maxwell stress. The electrical actuation mechanism can result in much higher operating efficiency (ratio of mechanical work to input electrical energy) and faster actuation speed than those of LCEs (8,9). Besides functioning as soft linear actuators, DE actuators could be applied...

Contact electrification between water and a solid surface is crucial for physicochemical processes at water–solid interfaces. However, the nature of the involved processes remains poorly understood, especially in the initial stage of the interface formation. Here we report that H 2 O 2 is spontaneously produced from the hydroxyl groups on the solid surface when contact occurred. The density of hydroxyl groups affects the H 2 O 2 yield. The participation of hydroxyl groups in H 2 O 2 generation is confirmed by mass spectrometric detection of 18 O in the product of the reaction between 4-carboxyphenylboronic acid and 18 O–labeled H 2 O 2 resulting from 18 O 2 plasma treatment of the surface. We propose a model for H 2 O 2 generation based on recombination of the hydroxyl radicals produced from the surface hydroxyl groups in the water–solid contact process. Our observations show that the spontaneous generation of H 2 O 2 is universal on the surfaces of soil and atmospheric fine particles in a humid environment.