Among the various inexpensive and environmentally friendly candidates for supercapacitors, MnO₂ has been investigated extensively due to its outstanding comprehensive performance. However, MnO₂ exhibits poor cycle performance in many research work, which may possibly due to the dissolution of MnO₂ during charge-discharge cycles. In this work, we report a facile and effective method to improve the cycle performance of MnO₂ films by ultrasonic-assisted electrodeposition.

The MnO₂ films were electrodeposited on Ni foam substrates without treatment (denoted as sample I), with post-ultrasonic treatment (denoted as sample II) and synchro-ultrasonic treatment (denoted as sample III). The structures of the MnO₂ films are characterized by SEM and TEM. MnO₂ nanowire clusters and nanosheet arrays can be observed in the samples prepared by post-ultrasonic assisted and synchro-ultrasonic assisted electrodeposition, respectively, while irregular nanosheets are inspected in the samples without ultrasonic assistance. The electrochemical testing of cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) measurements show typical pseudocapacitive behaviors for all samples. Directly electrodeposited MnO₂ film without ultrasonic assistance suffers from obvious attenuation in capacitance, which decreases about 37% from 230 F g^-1 in 3000 cycles. Comparatively, the ultrasonically treated MnO₂ films exhibit a maximum specific capacitance of 369 F g^-1 and 325 F g^-1 at 1 A g^-1, respectively, which can maintain 98% and 86% up to 3000 cycles, respectively. The effects of the deposition current and ultrasonic time were also investigated. The enhancement in electrochemical performance is significant for the samples with ultrasonic-assisted treatment, suggesting the treated MnO₂ films to be a promising choice for supercapacitors.