Measured peak device strain D/L of cantilever-actuators in 1 M NaOH vs their Ni thickness tNi, with the Ni(OH)2/NiOOH layers deposited at 0.4 mA/cm2 for 30 min in all cases.
Effects of plating time on the thickness of Ni(OH)2 /NiOOH ta and device strain D/L, with thickness of bulk Ni = 1.9 µm. The plating current density was 0.4 mA/cm2 ,
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Illustration of how D and L are measured.
Top shows the schematic of nickel hydroxide/oxyhydroxide actuator at oxidized (actuated) and reduced states.
Nickel hydroxide/oxyhydroxide actuator
Ni(OH)2 <-> NiOOH is a well-known electrochemical reaction which takes place during the charging and discharging of nickel-hydride rechargeable batteries, which causes undesirable swelling and shrinkage of the electrodes and shortens the battery's service life. In the present work, we identified a promising use of the redox reaction to generate electrochemical actuation.
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Microstructure: The actuator is a bi-layer nickel hydroxide/bulk Ni foil (see left figure) of standard dimension 2cm x 0.4cm (can be modified). The actuator is synthesized via the electrodeposition of pure nickel onto fourine-doped-tin-oxide (FTO) electrode at a constant current density of -15mA/cm^2 for 10 minutes, followed by electrodeposition of nickel hydroxide at current density of 0.4mA/cm^2 for another 40 minutes under vigorous stirring.
The SEM images below indicates that the synthesized structure are two solid layers without nanopores. The microstructure is quite different from nanoporous nickel actuators we studied before since we would like to facilitate a different actuation mechanism. The charge-induced actuation would prevail under high surface-to-volume ratio of nanoporous structure, we have however discovered that the redox reaction results in much higher strain than charge-induced mechanism. The redox-reaction-induced strain is proportional to the volume of nickel hydroxide layer, and thus a solid structure would be more beneficial than a nanoporous one to increase macroscopic strain.
Actuation: The actuation is performend in an electrochemical cell consisted of the actuator as the working electrode, a platinum mesh as counter electrode, and a saturated calomel electrode as reference electrode (see below). When the actuator is scanned with positive potential, nickel hydroxide (Ni(II)) oxidizes into nickel oxyhydroxide (Ni (III)) and its unit cell volume reduces more than 10%, resulting in macroscopic bending/contraction of the actuator towards the direction of Ni(II) film. The reduction reaction of Ni(III) back to Ni(II) enables unit cell volume expansion and the recovery to actuator's initial position. This is opposite to nanoporous metals, where an application of positive potential results in expansion rather than contraction in metal. A large device strain (3.5mm end-deflection for a 5mm long actuator) of ~ 70% was identified, corresponding to an internal strain of ~ 0.16%. This value is comparable to nanoporous Au/Pt actuators, but is still lower than the state-of-the-art nanoporous gold actuator by Detsi et. al [2].
Desirable features as a compact artificial muscle:
(i) A low voltage is required to trigger actuation. Our operation voltage can be as low as 0.4V, which is much lower than that required by piezroceramics (> 1kV), dielectric elastomers, and nanoporous Au/Pt (~ 1.6V).
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(ii) Scalable actuation strain. We found that the end-deflection of the actuator is proportional to the thickness of the nickel hydroxide layer and also is inversely proportional to the thickness of bulk Ni. This is not surprising since the strain is proportional to the volumetric difference in Ni(OH)2 layer induced by redox reaction. Also, the thicker the bulk Ni, the higher the resistance that is imposed on actuation and therefore the smaller the strain. The device strain can thus be scaled to a range of desired magnitudes by manipulating the electroplating conditions to achieve a good balance between thickness of the two layers. This flexibility adds to the application values of nickel hydroxide.oxyhydroxide actuator.
(iv) The actuator can be easily fabricated into various 2D shape through masked-electrodeposition. The regions that do not want to be electrodeposited onto the electrode (flourine-doped-tin-oxide, FTO) can be covered by chemical-resistant sticker during electrodeposition, enabling a great variety of 2D shapes to be made to adapt to particular purpose. The following videos show actuation of circular-, branched-, and star-shaped actuators.
(v) The color indicates actuation state. The color of Ni(II) is light silver while the Ni(III) film appear in darker grey. The actuator can thus indicate its actuation state by the distinction of its color, implying some optical application values.
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Application: The advantages such as large tunable strain, compactness, low voltage requirement, and the flexibility to be produced into different shapes suggest that the actuator may be serve as prime-movers in liquid which mimic the shape of hydrodynamically efficient propellers. An undergraduate project was carried out to investigate the propulsion capability of the actuators.
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References:
[1] Kwan KW, Hau NY, Feng SP, Ngan HW. 2017. Electrochemical actuation of nickel hydroxide/oxyhydroxide at sub-volt voltages. Sensors and Actuators B, 248:657–664
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[2] Detsi E, Punzhin S, Rao J, Onck PR, and Jeff De Hosson TM. 2012. Enhanced Strain in Functional Nanoporous Gold with a Dual Microscopic Length Scale Structure. ACS Nano, 6 (5): 3734–3744
(iii) The actuator can be actuated as a compact single component in air. We packaged the actuators into a self-contained assembly as shown in the figure below. The assembly consists of two Nickel hydroxide/oxyhydroxide actuators, one acting as working electrode and the other as counter electrode. The alkaline electrolyte is dissolved in the liquid film sandwiched between the two actuators. The surface tension in the film holds the two actuators together. The assembly eliminates the need for bulky, 3-electrode electrochemical cell, and can also actuate in ambient dry air at a frequency as high as 25Hz.