Open-pore nickel (Ni) foams are characterized using surface science and electrochemical techniques. A scanning electron microscopy analysis reveals interconnected Ni struts that generate small and large pores of ca. 50 and 500 μm in size, respectively. An X-ray photoelectron spectroscopy (XPS) analysis of the surface-chemical composition of the Ni foams shows that there are oxidized and metallic sections within their surfaces despite being prepared by sintering in an oxidizing atmosphere at a high temperature and being stored in moist air. The ratio of the areas of oxidized and metallic sections is evaluated using XPS data. Chemical etching of the Ni foams results in removal of the native surface oxide/hydroxide without altering the three-dimensional structure; it also increases the roughness (R) of the surfaces of Ni struts giving rise to an increase in the electrochemically active surface area (Aecsa). Thermal treatment of Ni foams in an H2(g) atmosphere at 500 °C reduces the native surface oxide/hydroxide but does not increase R or Aecsa. Electrochemical behavior of the Ni foams is examined in 0.5 M aqueous KOH solution using cyclic-voltammetry (CV) and electrochemical impedance spectroscopy (EIS). As-received, chemically etched, thermally reduced and electro-oxidized Ni foams generate distinct CV profiles; their features are assigned to oxidized and metallic surface states. The observations made on the basis of XPS measurements are corroborated by the results of CV analyses. The application of CV and XPS or EIS allows in situ determination of Aecsa and the specific surface area (As) of the chemically etched and thermally reduced Ni foams. The values of As determined on the basis of joint CV and XPS measurements are 227 ± 74 and 149 ± 48 cm(2) g(-1) for the etched and reduced Ni foams, respectively. The values of As determined on the basis of CV, XPS and EIS measurements are 241 ± 80 and 160 ± 23 cm g(-1) for the etched and reduced Ni foams, respectively.

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Electro-oxidation of Ni(poly) in 0.5 M aqueous KOH solution at various polarization potentials (E p ) up to 0.5 V vs. reversible hydrogen electrode, for polarization times (t p ) up to 2 h, and at 285≤T≤318 K leads to the formation of a thin layer of α-Ni(OH) 2 . Interfacial capacitance measurements show that the Ni(poly) electrode covered with a layer of α-Ni(OH) 2 can be completely reduced back to its metallic state by applying a negativegoing potential scan with a lower potential limit of −0.2 V. An increase of E p , t p , and/or T results in an increase of the thickness of the α-Ni(OH) 2 layer, which, however, never exceeds two monolayers. The electrochemical formation of α-Ni(OH) 2 follows a direct logarithmic growth kinetic law. The results reported in this contribution and their interpretation imply that other oxide growth theories, such as the Langmuir-type adsorption, the point defect model, the electron tunneling, or the nucleation-and-growth model, are not applicable to the growth of α-Ni(OH) 2 . The potentiostatic growth of α-Ni(OH) 2 on Ni(poly) is successfully treated by applying the interfacial place-exchange mechanism and the associated kinetic law.

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