High-performance polymer actuators based on an iridium oxide and vapor-grown carbon nanofibers combining electrostatic double-layer and faradaic capacitor mechanisms

- We developed an actuator that uses an IrO2·2H2O/VGCF/IL gel electrode.
- The electrochemical and electromechanical properties of IrO2·2H2O/VGCF/IL actuators were examined.
- The electrode in this actuator system exhibited behavior similar to that of both an electrostatic double-layer capacitor and a faradaic capacitor.
- This actuator generated strain values that surpassed those of SWCNT-based actuators.
- The frequency dependence of the displacement response was successfully simulated using an electrochemical kinetic model similar to that used with SWCNT-based actuators.

The electrochemical and electromechanical properties of polymer actuators that leverage the synergistic effect from combining a noble metal oxide (iridium oxide dihydrate (IrO2·2H2O)), vapor-grown carbon nanofibers (VGCFs) and an ionic liquid (IL) in the electrode were compared with those of actuators prepared using VGCFs or single-walled carbon nanotubes (SWCNTs) without IrO2, or with an IrO2·2H2O/carbon black (CB)/IL composition. The electrode in this actuator system is equivalent to an electrochemical capacitor, and exhibits behavior similar to that of both an electrostatic double-layer capacitor (EDLC) and a faradaic capacitor (FC). The mechanism underlying the functioning of the IrO2·2H2O/VGCF/IL actuators which exhibit from both EDLC and FC mechanisms was found to be different from that for devices produced using VGCFs or SWCNTs alone (which exhibit only the EDLC mechanism) and using IrO2·2H2O/CB/IL, which exhibit from both EDLC and FC mechanisms, with the FC mechanism providing the largest contribution. An IrO2·2H2O/VGCF/EMI[BF4] actuator exhibited a maximum strain of 0.75%, a value approximately 1.8 times that obtained from a SWCNT-only actuator. This device also generated a maximum blocking force stress of 2.58 MPa (1.3 times that of a SWCNT-only actuator) and a maximum calculated stress of 0.66 MPa (2.2 times that of a IrO2·2H2O/CB/EMI[BF4] actuator).Although the frequency dependence of the displacement responses of an IrO2·2H2O/CB/IL polymer actuator was not successfully simulated using a double-layer charging kinetic model in previous work, this was found to be possible for the IrO2·2H2O/VGCF/IL actuators in the present study. Simulations of the electromechanical response of the IrO2·2H2O/VGCF/IL actuators correctly predicted strains at low frequencies as well as the associated time constants, confirming that the model is applicable to both EDLC-based actuator systems and the newly fabricated EDLC/FC system. These results suggest that flexible, robust films enabled by the synergistic effect obtained by combining noble metal oxides and VGCFs can have significant potential as actuator materials for wearable and energy-conversion devices.

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