Electrochemical mechanics of metal thin films : charge-induced reversible surface stress for actuation

Abstract

The intrinsic charge-induced surface stress of Ni thin films during electrochemical reactions with an alkaline electrolyte is measured in situ. Surface stresses induced by H absorption/desorption, α-Ni(OH)2 formation, capacitive double-layer charging, the α- to β-Ni(OH)2 transformation, and β-Ni(OH)2/β-NiOOH redox reactions are identified, and each provided additive contributions to the overall stress state. Surface stresses are magnified in high-surface-area nanoporous Ni because local stress-relaxation mechanisms are restricted when compared to a smooth Ni film. Ni film reversible tensile/compressive surface stresses correlate with anodic/cathodic potential scanning but with an opposite trend to that of a less reactive Au film. Surface stresses in the Ni films are up to 40 times that of Au films and suggest the possibility of using controlled surface-stress generation for electrochemical actuation.