The Origin of Electrochemical Actuation of MnO ₂ /Ni Bilayer Film Derived by Redox Pseudocapacitive Process

Pseudocapacitance-induced electrochemical actuators (EC-actuators) have attracted great attention in robots and artificial intelligence technologies. Despite major efforts to design such EC-actuators, a molecular-level understanding of the deformation mechanism is still lacking. Here, a reversible deformation of a freestanding MnO ₂ /Ni bilayer film is demonstrated and in situ electrochemical atomic force microscopy, in situ Raman spectroscopy, and density functional theory simulation are used to study the origin of the deformation. The results show that the electrochemical actuation of the MnO ₂ /Ni film is highly related with the redox pseudocapacitive behavior of MnO ₂ layer. Valence state variation of Mn element, shortening and lengthening of MnO bond, and insertion and extraction of Na ⁺ ions, which all result from the redox pseudocapacitance of MnO ₂ during charging and discharging, eventually lead to the reversible contraction and expansion of MnO ₂ morphology. Such action counters with the nonactive Ni layer, finally inducing the reversible deformation of the MnO ₂ /Ni bilayer film. It is believed that the study can provide useful guidance to design better EC-actuators in the future.