An Anionic Framework-Enabled Ion-Anchoring Strategy for Frequency-Tolerant Electrochemical Actuation

Artificial muscles, owing to their lightweight nature, mechanical compliance and the ability to emulate biological muscle actuation, demonstrate immense potential in the field of bionic intelligent robotics. However, existing ionic electrochemical actuators generally exhibit pronounced frequency dependence and unstable actuation outputs under variable-frequency operation, which severely limit their ability to simulate scenarios requiring stable performance across varying frequencies. Herein, we report a zinc tetraphenylporphyrin (Zn-TPP)/1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) electrolyte membrane, in which the anionic TPP^2- framework effectively anchors EMIm⁺ cations. This effect restricts cation migration under applied electric fields and thereby stabilizes the charge distribution within the electrolyte membrane. Benefiting from this mechanism, the actuator achieves an outstanding 99.97% displacement retention over a broad frequency range of 0.1-5 Hz at 3 V. In contrast, a conventional EMImBF₄/PVDF-HFP-based electrochemical actuator exhibits a displacement retention of only 25% at 5 Hz. By comparing devices with three different TPP^2--containing electrolyte membranes, we confirm a positive correlation between the electrostatic potential of TPP^2- and actuator displacement retention. A higher electrostatic potential enables stable operation over a broader frequency range, whereas a lower electrostatic potential reduces stability but yields larger bending displacement, thereby allowing a tunable balance between stability and displacement amplitude to meet diverse application needs. In addition, the actuator maintains stable performance over a relative humidity range of 40-70%, ensuring reliable operation in biofluid-like environments. Notably, the actuator mimics the contraction behavior of the human Achilles tendon and maintains stable, repeatable responses under varied rhythmic conditions, highlighting its potential for next-generation soft robotics.