Existing nanoporous metallic actuators are mostly based on Au, Pd, or Au-Pt alloys due to the ease to form mutually soluble precursor material for de-alloying synthesis, and their small accessible ligament sizes which enables them to have a larger surface areas compared to other nanoporous metals for actuation to take place. One characteristic of these precious metals is that they do not react easily with acids and alkalines, and thus can preserve "clean" metal surfaces in electrolytes when actuated in the electrochemical cell. When an external voltage is applied to the actuator, the ions in the electrolyte are attracted and "stored" at the large surface of nanoporous metal, affecting the strength of intermetallic bonds at the surface. This results in changes in stress state at the surface and also in the bulk, which results in actuation. However, the nanoligaments of Au/Pt actuators were found to coarsen in electrolytes over time, reducing the surface area available for actuation.

 

As Hakamada et.al [1] cites Kramer et. al [2] in his work, "the dependence of surface stress on the surface charge density for a clean metal surface has been known as electrocapillarity", and this phenomenon in principle can be found in all metals. It is of scientific and also commercial interests to seek alternative metals which do not suffer substantially from ligament coarsening effect.

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Nickel is an abundant metal with a lower cost compared to gold and platinum. Nanoporous nickel has also been found to exhibit actuation behaviour similar to nanoporous Au/Pt in electrolytes. Hakamada et. al [1], for example, observed stable actuation in nanoporous/bulk Ni stacked bilayer actuated in 1 M NaOH when the potential between electrodes is switched between +/- 1 V. 

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Carrying on with this line of research, we published results in 2015[3] and 2016[4] on the actuation behaviour of nanoporous nickel with two distinguished surface textures. Contrary to the work by Hakamade et. al [1] in which the nanoporous nickel is synthesized by de-alloying method, we used "inverse template" method to produce highly structured nanohoneycomb and nano-wire forest surfaces. We found that the straight porous channels in both nanostructures help maintain good ion transport which improves actuation rates. The actuation strain was also improved, possibly because the well-aligned nanopore channels results in less stress cancellation during actuation than disordered nanoligamnets synthesized from de-alloying method. In addition, the ligaments almost did not coarsen over time due to a thin protection layer of nickel hydroxide formed at the surface of nanoporous nickel while immersed in sodium hydroxide (NaOH).

 

We identified different actuation mechanisms at low and high actuation rates through electrochemical impedance spectroscopy. We confirmed our hypothesis that the actuation strain which only prevails at frequency < 0.83 Hz is due to the volumetric changes induced by redox reaction in nickel hydroxide thin film formed at the surface of nanoporous nickel. This actuation mechanism is different from the capacitive charging/discharing of electrochemical-double-layer mechanism as for existing nanoporous Au/Pt actuators. In order to maximize the strain due to redox reaction, we developed actuators consisted of purely solid bi-layer nickel hydroxide/Ni bulk, and we obtained a large device strain up to 70% and actuation rates as high as 25 Hz in our work published in 2017 [5].

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