Chuan Cheng* and Alfonso H. W. Ngan
ARTICLE
Reversible Electrochemical Actuation of Metallic Nanohoneycombs Induced by Pseudocapacitive Redox Processes Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, People's Republic of China
ABSTRACT Current metallic-based electrochemical actuators are limited to nanoporous
gold/platinum with randomly distributed pores, where the charge-induced reversible strain is mainly due to the nonfaradic charging/discharging processes along the capacitive electrochemical double layer. Here, we report an electrochemical actuating property of nanohoneycomb-structured 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 bilayered cantilever, comprising a nanohoneycomb 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. The maximum strain that can be achieved under a certain scan rate complies with a linear relationship with the capacity. Benefiting from the stable Ni(II)/Ni(III) redox couples at the electrode surface, the reversible actuation is very stable in hydroxide solutions. KEYWORDS: nanohoneycomb nickel . electrochemical actuation . surface-charge-induced strain . reversible redox reactions . electrochemical double layer
H
uman skeletal muscles are ideal actuators with large strain amplitudes (>40%), fast strain response (>50%/s), high energy efficiency (∼40%), and durability (>109 life cycles).1 Materials that can mimic the properties of muscles upon outside electrical/chemical triggering are known as “artificial muscles”, which may serve, for instance, as the building blocks of robots.25 Nowadays, piezoceramics, electroactive polymers, and shape memory alloys have already been commercialized as actuation materials for artificial muscle applications.6,7 However, their use is limited by factors including ultrahigh actuation voltages needed (>1 kV for both piezoceramics and electroactive polymers), small strain amplitudes (∼0.1% for piezoceramics), and low energy efficiency (∼3% for shape memory alloys).1,8 In the past decade, increasing attention has been paid to a diverse group of nanoporous materials for electrochemical actuations, including nanoporous noble metals,913 carbon nanotubes,1,3,1420 conducting polymers,1,3 carbon aerogels,16,21 and graphene films,2224 to transform electrical energy into mechanical energy for potential CHENG AND NGAN
applications as artificial muscles.25 Due to the low triggering voltages for the electrochemical process in the actuation mechanism, these nanoporous materials require voltages on the order of 1 V to actuate, which is about 3 orders of magnitude lower than that for conventional actuation materials,6,7 such as piezoceramics, which require high voltages to realize dipole alignment in the bulk, while the actuation strain achievable is comparable to, or even 1 order of magnitude higher than, these conventional actuation materials. In spite of the above, many challenges still need to be overcome before nanostructured materials can be widely used in practical actuation applications.15,26 Although polymer-based actuation materials have large charge-induced reversible strains (>100%), they suffer from slow response times21,27 and low stiffness and strengh.9 Metallic-based nanoporous materials are competitive candidates for electrochemical actuators in terms of their much better stiffness and strength. Furthermore, by decreasing the ligament size in these nanoporous metals, not only can the yield strength and stiffness be enhanced,2830 VOL. XXX
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* Address correspondence to
[email protected]. Received for review December 30, 2014 and accepted March 10, 2015. Published online 10.1021/nn507466n C XXXX American Chemical Society
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ARTICLE Figure 1. Schematic of the fabrication process of an ordered nanohoneycomb Ni layer backed by a solid Ni layer. (a) AAO template; (b) Au sputtering on the top surface of the AAO template; (c) polymer mounting into the AAO template; (d) inverse polymer template; (e) electrochemical deposition of Ni into the polymer template; (f) 3-D view of the bilayered nanohoneycomb/solid Ni foil.
but the actuation strain can also be increased because the strain is proportional to the surface-area-to-volume ratio.12 However, the nanoporous metals that have been studied to date have certain limitations. First, these metallic-based electrochemical actuators are restricted to noble metals, such as Pt,12 Au,5,9 and AuPt composites,9 because of their unique properties including the high redox potentials and small accessible ligament size (
