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.

Surface Science and Electrochemical Analysis of Nickel Foams

The electrochemical cyclic voltammetric (CV) oxidation behavior of an arc-derived single-wall carbon nanotube (SWNT) sample in potassium hydroxide solution was investigated. Amorphous carbon in the as-grown SWNT sample was effectively removed by the CV oxidation, as confirmed by analyzing the sp(3)/sp(2) carbon ratio from C1s XPS spectra and HRTEM observations. The removal of the amorphous carbon led to the exposure of metal nanoparticles, hence facilitating the elimination of the metal impurities by subsequent HCl washing. The CV oxidation can be applied as an alternative oxidative treatment for the purification of SWNT samples. Redox peaks were observed during the CV oxidation. The reduction peaks in the range of -0.96 to -1.0 V and the oxidation peaks in the range of -0.61 to -0.49 V were attributed to the electrochemical redox transformations between metallic Fe and Fe(II) oxide, Fe(II) oxide and Fe(III) oxide, as well as Ni and Ni(II) oxide, and the observed reduction peaks in the potential range of 0.29-0.13 V were believed to be caused by the electrochemical reduction of NiOOH into Ni(OH)(2). The intensity of all of the redox peaks was dependent on the cycle number because more and more metal nanoparticles could be exposed as a result of the incremental removal or damage of the amorphous carbon coating during the CV oxidation, while the intensity remained almost unchanged after 80 cycles because of the completion of the amorphous carbon removal. Therefore, the redox peaks from the electrochemical redox reactions of Fe and Ni impurities can be considered as a benchmark for the removal extent of the amorphous carbon, and optimal electrochemical oxidation time for the purification of the as-grown SWNT sample can be determined in real time during the CV oxidation treatment. This is a predominant advantage of the CV oxidation over common oxidation methods using air or other oxidizing reagents for SWNT purification.

Purification of single-wall carbon nanotubes by electrochemical oxidation

Electrocatalytic oxidation of cyclohexanol on a nickel oxyhydroxide modified nickel electrode in alkaline solutions

On the electrocatalytic urea oxidation on nickel oxide nanoparticles modified glassy carbon electrode

“Small” carbon nano-onions (CNOs) are spherical, ca. 5 nm in diameter, concentric shells of graphitic carbon that can be also described as multi-shelled fullerenes. Given the easy functionalization and high thermal stability of the CNOs produced from nanodiamond, they are the most obvious choice for studying the potential applications of these multi-shelled fullerenes in electrochemical supercapacitors (ES). Since limited accessibility of the carbon surface to electrolyte penetration is observed for carbon nano-onions, performance enhancement was accomplished by modifying the CNO surfaces with pseudocapacitive redox materials: Ni(OH)2 and NiO. These composites were characterized by TEM, SEM, XRD, TGA-DTG-DTA and Raman spectroscopy. The electrochemical properties of these composites were also investigated. Compared with pristine CNOs (30.6 F g−1 at 5 mV s−1), modified CNOs (1225.2 F g−1 for CNOs/Ni(OH)2 and 290.6 F g−1 for CNOs/NiO, both at 5 mV s−1) show improved electrochemical performance, promising for the development of supercapacitors.

M. Dmochowska

Papers

Electrochemical behavior of nickel deposited on reticulated vitreous carbon