RESUMEN
Poor rate capability, the biggest barrier to potential applications of electrochemical actuators (ECAs), is primarily resulted from symmetric electrochemical reactions. This makes it extremely difficult for ECAs to actuate above 1 Hz while maintaining sufficient displacement retainability compared with their actuations at relatively low frequencies, particularly when working in liquids. Here, tungsten trisulfide (WS3) assisted tungsten disulfide nano onions are synthesized through a one-step laser-assisted strategy. Using the irreversibility of WS3 in adsorbing hydrogen in an acidic solution, the electrochemical reaction of tungsten sulfide nano onions is tailored to realize an asymmetric redox reaction for breaking the symmetry of the electrical double layer and battery-like process. Experiments demonstrate that the ECA's response rate (0.24 mm-1 s-1) is at least 10 times faster than that of the previously reported ECAs. Moreover, this ECA can actuate at 30 Hz and reaches top performance in liquids at 4 Hz with long-term durability (>90% after 23 000 cycles), which is comparable to that of electromagnetic and electrothermal actuators. To understand the electrochemical actuation of tungsten sulfide from the atomic scale to the macroscopic scale, density functional theory calculations are conducted and an electrochemomechanical coupling model is proposed. A new generation of subvolt electric-driven actuators used in underwater robotics can be developed by modulating the electrochemical response and chemomechanical coupling effect.
RESUMEN
Electrochemical strain microscopy (ESM) is a powerful tool to resolve ionic transport and electrochemical processes with a nanoscale resolution. To ascertain the underlying mechanism that governs the signal generation of ESM imaging, a fully coupled nonlinear electrochemomechanical model based on the finite element method is developed and applied to LiMn2O4 particles. The frequency dependence of the ESM response, in particular the response at high frequencies used in the detection regime, is investigated in detail. The performed analysis demonstrates that the error induced by the decoupling approximation increases with decreasing bias frequency due to the relatively large variation in ion concentration. In the high frequency regime, the results reveal that the stress effect is negligible and local electroneutrality holds, providing the simplification of numerical simulation for ESM imaging. By applying an alternative current voltage, we suggest that the detectable signal observed in ESM imaging can be attributed to the Vegard effect, which was controversial in previous linear models. The local distribution of ion concentration shows that the ionic reorganization only takes place near the tip-surface junction, the spatial extent of which can be described by two relevant lengths, the contact radius and ion drift length, which determine the spatial lateral resolution and depth resolution, respectively, in ESM imaging. Through a parametric study, the electromigration is proved to be dominant at high frequencies and the relationship between ESM amplitude and some parameters may offer a strategy to measure local electrochemical reactivity. The impact of contact force is evaluated and the results indicate that the local compression reduces ion concentration and the resultant ESM signal in the detection regime. Thus attention must be paid to the contact force when a comparison between different measurements is conducted. The combination of the numerical model and experiment holds the promise of quantitative probing of local electrochemical parameters in solids.