Conductive polymer (e.g., polyaniline and polypyrrole)-based ECAs can generate high actuation strains due to large volume change induced by the doping/dedoping during the faradaic charge and discharge process, [3] but usually show limited cycling stability and decreased response under high frequencies/rates. [1] Many conductive inorganic materials have also been studied as active materials for ECAs, including 0D metal nanoparticles (e.g., Au and Pt) and carbon black, [4][5][6] 1D carbon nanotubes and nanofibers (CNTs and CNFs), [1,[7][8][9] as well as 2D graphene, [10][11][12][13][14][15] graphdiyne, [10] black phosphorous (BP), [32] and MoS 2 . [16] However, 0D metal nanoparticle and carbon-black-based electrodes show limited specific capacitance, poor toughness, and low stiffness, limiting the deformation, cycling life, and output force. 1D CNT and CNF-based electrodes can have better mechanical performance but show small volumetric capacitance restricting the actuation strain. Atomically thin 2D nanomaterial (such as graphene)-based electrodes can have significantly higher specific capacitance while maintaining a good mechanical strength and modulus, hence they are very promising for fabricating high-performance ECAs.Recently, 2D Ti 3 C 2 T x (T = OH, O, and F) MXene derived from Ti 3 C 2 Al has also attracted researchers' attention for ECA applications, [17][18][19][20] due to its ultrahigh electrical conductivity, [21] high mechanical performance, [22] and large volumetric specific capacitance. [23,24] Come et al. found that a Ti 3 C 2 T x paper undergoes a large contraction along the thickness direction during charge in a Li 2 SO 4 solution, accompanied by a ≈2.1% shrinkage of layer distance between MXene sheets. [20] This large volume change during charge and discharge renders MXene as a good active material for ECAs. Pang et al. showed that a MXenebased ECA with a poly(vinyl alcohol) (PVA)/H 2 SO 4 gel electrolyte generated a peak-to-peak strain difference of 0.26% under a triangular wave voltage of ±1 V and 1 mHz, which was ascribed to the expansion and shrinkage of the interlayer spacing during charge and discharge. However, the actuation at high frequency is undesirable due to limited ion diffusion across the film. Wang et al. fabricated a 3D structured polystyrene (PS) microsphere/MXene composite-based ECA, which generated a peakto-peak strain difference of 1.18% under a square wave voltage of ±1.5 V and 0.1 Hz due to the much enhanced ion diffusion by the PS. However, the introduction of PS microspheres decreased the tensile strength and Young's modulus of the composite and the resulting ECA. In these cases, the Young's Ti 3 C 2 T x MXene film is promising for electrochemical actuators due to its high electrical conductivity and volumetric capacitance. However, its actuation performance is limited by the slow ion diffusion through the film and poor mechanical property in aqueous electrolytes. Here, molecular-level methylcellulose (MC)/MXene hybrid films are assembled with obviously enlarged layer dis...

Porous carbon black/reduced graphene oxide hybrid fiber was fabricated by a scalable wet-spinning method and a flexible solid-state SC with excellent electrochemical performance was assembled using the hybrid fiber.

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Soft electrical actuators driven by low voltages are promising for interactive human-machine interfaces (iHMI) applications including executing orders to complete various tasks and communicating with humans. The attractive features of low-voltage soft electrical actuators include their good safety, low power consumption, small system size, and nonrigid or deformable characteristics. This review covers three typical classes of electrical actuators, namely, electrochemical, electrothermal, and other electrical (dielectric, electrostatic, ferroelectric, and plasticized gel) actuators according to their mechanism and working potential range. For each kind of actuators, the advantages, working principle, device configuration/design, materials selection, recent progress, and potential applications for iHMI are summarized, with the strategies for enhancing the actuation performance under low voltages being highlighted. Finally, the challenges for those soft actuators and possible solutions are discussed.