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

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