Electrochemical and AFM study of nickel nucleation mechanisms on vitreous carbon from ammonium sulfate solutions

The role of different inorganic anions on passive film durability for nickel against chloride ions pitting corrosion in borate buffered solution

Potentiodynamic and cyclic voltammetric studies were carried out on nickel in borate buffered saline (pH = 8.49). The anodic excursion spans of nickel in borate buffer solution do not involve active/passive transition. The passive film starts to break down in the presence of Cl-ions, which causes pitting damage. The data reveal that the increasing Cl− concentration and solution temperature shifts the E pit to the active direction while the increasing in scan rate shifts the E pit to the positive direction. The pitting potential (E pit) shifted in a positive direction when increasing concentrations of Wo4 −2 and MoO4 −2 anions were added to a borate buffer solution containing Cl− ions, showing that the additional anions had an inhibitory influence on the pitting corrosion. While the NO3 − anion is ineffectual as an inhibitor and rather speeds up pitting corrosion, the NO2 − anion has a slight inhibitory impact on pitting corrosion.

Electrode Surface Coverage with Deposit Generated Under Conditions of Electrochemical Nucleation and Growth. A Mathematical Analysis

The theory of the diffusion limited electrochemical nucleation and growth of a deposit consisting of isolated 3D hemispherical nuclei has been re-analysed. The analysis focuses on a widely discussed model which assumes formation of “diffusion zones” around the growing nuclei. It has been proposed in the literature that the deposit-free fraction of the surface area of the substrate can be directly calculated from the substrate coverage with the “diffusion zones”. The aim of this work is to analyse whether such an approach can be applied for the growth of isolated 3D hemispherical nuclei. This is accomplished by evaluation of equations which describe nuclei radii at various stages of the deposition process. The formulae allow determining the substrate surface coverage with the growing deposit. This, in turn, allows simulating and analysing faradaic currents due to other than the electrodeposition reactions which take place at the deposit-free fraction of the substrate surface. Both instantaneous and progressive modes of the nucleation are discussed and the influence of the nucleation type on the faradaic currents is outlined. A comparison with other approaches reported in the literature indicates that the deposit-free fraction of the substrate surface may not always be determined by means of recalculation of the substrate coverage with the “diffusion zones”.

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Insights into Electronucleation and Electrodeposition of Nickel from a Non-aqueous Solvent Based on NiCl2·6H2O Dissolved in Ethylene Glycol

This work deals with nickel electronucleation and growth processes onto a glassy carbon electrode from NiCl₂·6H₂O dissolved in ethylene glycol (EG) solutions with and without 250 mM NaCl as a supporting electrolyte. The physicochemical properties of EG solutions, namely, viscosity and conductivity, were determined for different Ni(II) concentrations. From cyclic voltammetry, it was found that in the absence of the supporting electrolyte, the cathodic efficiency of Ni electrodeposition is about 88%; however, in the presence of the supporting electrolyte, the cathodic efficiency was reduced to 26% due to water (added along the supporting electrolyte) reduction on the growing surfaces of Ni nuclei. This side reaction produced both H_2(g) and OH^- ions. Part of the former was occluded in Ni, and the latter reacted with Ni(II) ions in EG forming passivation products such as Ni(OH)_2(s). Moreover, it was shown that metallic Ni did not catalyze the EG reduction in this system. From chronoamperometry, it was shown that in the absence of the supporting electrolyte, the amount of Ni electrodeposits, for the same overpotential and time, was higher than in the presence of the supporting electrolyte. The j-t plots recorded in the latter system, for different Ni(II) concentrations, were analyzed using a model which involves a contribution due to multiple 3D nucleation and diffusion-controlled growth and another related to the simultaneous reduction of water on the Ni nuclei growing surfaces. This model allows not only the quantification of the Ni nucleation kinetic parameters but also the effective deconvolution of the individual contributions to the total current; thus, from the integration of the j-t plots of these contributions, it was demonstrated that the charge amount of each process depends on the Ni(II) concentration. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy revealed the presence of pure Ni nanoparticles electrodeposited on the electrode surface. Moreover, X-ray measurements verified the formation of a high-crystallinity face-centered cubic structure with preferred orientation growth on the ⟨111⟩ direction, which were also corroborated by the magnetic measurement performed in a physical property measurement system.

Electrochemical deposition of nickel from aqueous electrolytic baths prepared by dissolution of metallic powder

A new method of preparation of aqueous electrolyte baths for electrochemical deposition of nickel targets for medical accelerators is presented. It starts with fast dissolution of metallic Ni powder in a HNO3-free solvent. Such obtained raw solution does not require additional treatment aimed to removal nitrates, such as the acid evaporation and Ni salt precipitation-dissolution. It is used directly for preparation of the nickel plating baths after dilution with water, setting up pH value and after possible addition of H3BO3. The pH of the baths ranges from alkaline to acidic. Deposition of 95% of ca. 50 mg of Ni dissolved in the bath takes ca. 3.5 h for the alkaline electrolyte while for the acidic solution it requires ca. 7 h. The Ni deposits obtained from the acidic bath are physically and chemically more stable and possess smoother and crack-free surfaces as compared to the coatings deposited from the alkaline bath. A method of estimation of concentration of H2O2 in the electrolytic bath is also proposed.

Electrochemical deposition of nickel targets from aqueous electrolytes for medical radioisotope production in accelerators: a review

This paper reviews reported methods of the electrochemical deposition of nickel layers which are used as target materials for accelerator production of medical radioisotopes. The review focuses on the electrodeposition carried out from aqueous electrolytes. It describes the main challenges related to the preparation of suitable Ni target layers, such as work with limited amounts of expensive isotopically enriched nickel; electrodeposition of sufficiently thick, smooth and free of cracks layers; and recovery of unreacted Ni isotopes from the irradiated targets and from used electrolytic baths.

Electroplated Silver-Nickel Core-Shell Nanowire Network Electrodes for Highly Efficient Perovskite Nanoparticle Light-Emitting Diodes

The low sheet resistance and high optical transparency of silver nanowires (AgNWs) make them a promising candidate for use as the flexible transparent electrode of light-emitting diodes (LEDs). In a perovskite LED (PeLED), however, the AgNW electrode can react with the overlying perovskite material by redox reactions, which limit the electroluminescence efficiency of the PeLED by causing the degradation of and generating defect states in the perovskite material. In this study, we prepared Ag-Ni core-shell NW electrodes using the solution-electroplating technique to realize highly efficient PeLEDs based on colloidal formamidinium lead bromide (FAPbBr₃) nanoparticles (NPs). Solvated Ni ions from the NiSO₄ source were deposited onto the surface of AgNW networks in three steps: (i) cathodic cleaning, (ii) adsorption of the Ni-ion complex onto the AgNW surface, and (iii) uniform electrodeposition of Ni. An ultrathin (∼3.5 nm) Ni layer was uniformly deposited onto the AgNW surface, which exhibited a sheet resistance of 16.7 Ω/sq and an optical transmittance of 90.2%. The Ag-Ni core-shell NWs not only increased the work function of the AgNW electrode, which facilitated hole injection into the emitting layer, but also suppressed the redox reaction between Ag and FAPbBr₃ NPs, which prevented the degradation of the emitting layer and the generation of defect states in it. The resulting PeLEDs based on FAPbBr₃ NPs with the Ag-Ni core-shell NWs showed high current efficiency of 44.01 cd/A, power efficiency of 35.45 lm/W, and external quantum efficiency of 9.67%.

Ammonia Generation via a Graphene-Coated Nickel Catalyst

A novel graphene-coated Ni electrode was developed in this investigation to improve corrosion resistance while unexpectedly enhancing the ammonia generation rate in the electrochemically induced urea to ammonia (eU2A) process, which is an electrochemical onsite ammonia generation method. The development of the electrode is crucial for the eU2A reactions since in the ammonia generation process, the concentration of ammonia is inevitably high on the surface of the electrode, leading to severe corrosion of the electrode and the loss of generated ammonia as well. In this paper, the graphene was derived from raw coal by using the chemical vapor deposition method and self-lifted onto a Ni electrode to form a protective layer for corrosion prevention. Transmission electron microscopy showed the synthesized graphene had few-layers and Raman spectroscopy indicated that the coating of graphene was stable during the eU2A reaction. As a result, the ammonia corrosion of the Ni electrode was dramatically reduced by ~20 times with the graphene coating method. More importantly, a higher ammonia generation rate (~2 times) was achieved using the graphene-coated Ni working electrode compared to a bare Ni electrode in the eU2A process.

Patterned Ni-P alloy films prepared by "reducing-discharging" process and the hydrophobic property

Patterned hydrophobic Ni-P alloy films consisting of orderly and regular micro-nanoscale particles were fabricated through the synergistic effect of electrochemical deposition and chemical deposition. Ni-P alloy films were deposited for different times and characterized by scanning electron microscope (SEM). It was confirmed that the addition of reducing agent induced the formation of nanoscale particles, in contrast with pure Ni film deposited by single electrochemical deposition. As "point-discharge effect", the current density was higher at the edge of the nanoscale particles, and Ni ions would be deposited at the particles through the "point-discharge effect". Then the Ni-P alloy films grew by "reducing-discharging" process. The X-ray photoelectron spectroscopy (XPS) was used to detect the composition and valence states of these alloy films. The existence of oxidation state of element P in these films corresponding to that in H2PO3(-), also gave direct evidence for the occurrence of chemical deposition, during the electrochemical deposition process. The prolongation of deposition time could provide more time for the patterned morphology to grow up. The surface roughness, evaluated by surface profilometer, increased as the deposition time extension. And these films showed gradually increased hydrophobic properties with the increase in deposition time.

NUCLEATION AND STRUCTURAL PROPERTIES OF NICKEL FILMS ELECTRODEPOSITED FROM, CHLORIDE AND SULFATE BATHS

Nickel films electrodeposited from chloride and sulfate baths at pH 3.8 have been investigated. The influence of the plating baths on the electrochemical growth and the characteristics of nickel were studied by means of cyclic voltammetry, potentiostatic steps (chronoamperometry), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. The electrocrystallization mechanism was analyzed using the Scharifker and Hills model. The nucleation mechanism was found to be progressive at -1.1 V versus SCE, while at elevated overpotentials (more negative than -1.2 V versus SCE) instantaneous nucleation behavior was obtained. AFM characterization of the deposits indicated that the baths composition influences greatly the morphology of the deposits. XRD analysis indicated polycrystalline growth of the Ni film with a preferred (111) orientation with the fcc structure for both baths. The Ni crystallite sizes are 19–31 nm for the sulfate bath and 14–33 nm for the chloride one.