Nickel plates of 12 mm diameter and 1 mm thickness (Goodfellow, 99.0% purity) were used as substrate. After polishing with 0.5 μm alumina paste (MicroPolish Alumina, Buehler), they were cleaned sequentially in an ultrasonic bath with acetone, isopropanol and Millipore water for five minutes each. Next, a thin layer of Ni(OH)2 was formed by electrochemical conditioning, as described in the next section.