Redox induced actuation in nanoporous nickel

Abstract

Charge-induced reversible straining was recently observed in nanoporous noble metals, such as Pt, Au, and Au-Pt alloys, which are becoming a promising type of electrochemical actuators for potential applications such as artificial muscles. These nano-porous metals, however, are expensive noble metals made from costly processes such as de-alloying. In this talk, we report an electrochemical actuating property of nanoporous nickel, with the actuation mechanism mainly due to a pseudocapacitive behavior by means of reversible faradic redox reactions. By using a dual-template synthesis method, a bi-layered cantilever, comprising a nanoporous layer backed by a solid layer of the same metal, was fabricated. Reversible bending of the cantilever upon cyclic potential triggering was observed. The strain of the cantilever increases nonlinearly with both potential and charge due to redox reactions. Benefiting from the stable Ni(II)/Ni(III) redox couples at the electrode surface, the reversible actuation is very stable in hydroxide solutions. Also, by conditioning the nanostructure of the actuating Ni into a dual-scale nanowire network that facilitates ion transport, record high strain response time in the order of 0.1 second was obtained, which is more than two orders faster than current metallic based actuators. To understand the mechanism, a multi-scale, multi-field simulation approach is used to model the above electrochemical actuation behavior. Specifically, molecular dynamics simulations with reactive force-field potentials and a modified charge-equilibrium (QEq) method are used to calculate the surface stress built up in Ni(100) surface in contact with water electrolyte due to a voltage applied across the interface, as a result of capacitive charging of the double layer in the contacting electrolyte as well as redox reaction of the Ni surface. The calculated surface stress is then used as input in a meso-scale finite-element (FE) model to compute the actuating stress set up in a single hexagonal unit cell of a Ni nanohoneycomb structure. The single-unit actuating stress is eventually used in a continuum FE model at a larger scale, to calculate the bending of an entire bilayered cantilever which replicates experimental conditions. The actuation deflection of the bilayered nanohoneycomb nickel is predicted to be 41.4 μm at 0.43 V vs the point of zero charge (PZC), which corresponds to ~0.48 V vs saturated calomel electrode (SCE), and this is in excellent agreement with the experimental value of 45-62 μm at a similar voltage vs SCE. This is the first successful attempt to simulate the electrochemical actuation of a real-sized, nano-porous metallic structure in an electrolytic environment.