In Situ Studies of the Passivation and Anodic Oxidation of Cobalt by Emission Mössbauer Spectroscopy: I . Theoretical Background, Experimental Methods, and Experimental Results for Borate Solution (pH 8.5)

Abstract

This paper is the first in a series of three that describe the application of emission Mössbauer spectroscopy to the in situ study of the cathodic and anodic behavior of cobalt. A brief review of the emission technique is given, along with a review of the chemical effects and Auger “after effects” that are associated with this experimental method. The surface sensitivity required to study thin anodic films was obtained by electrodepositing from 20 to 200Å thick Co57 active layers on cobalt surfaces. Emission Mössbauer spectra from Fe57 daughter (“probe”) atoms were obtained during polarization (vs. SCE) of specimens in buffered borate, pH 8.5. The specimens were free of surface oxide during cathodic polarization at −1100 mV. The spectra of specimens polarized at low passivating potentials, −100 mV, indicated the presence of both +2 and +3 charges. The +3 oxidation state of the Fe57 probe in the anodic film formed at this potential may arise from Auger after effects or chemical effects rather than from +3 states of the parent Co57. At higher passivating potentials +200 and +500 mV only the +3 oxidation state was observed. The existence of +2 oxidation state cannot be ruled out, however, because of the possibility for stabilization of defect charge states. Evidence for the +4 oxidation state in addition to +3 was found for specimens polarized at potentials in the transpassive range, +800 mV.

Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction

“…91,94 Some authors even reported the formation of Co(IV) species on the outer surface of the electrode. 91,126,127 The catalytic activity of cobalt depends on the oxide composition and on the electrochemical pre-treatment of the metal electrode. 91 The OER generally shows two overpotential regions 9,91,[128][129][130] with different Tafel slopes with the most reported slope at low overpotentials 42 mV dec −1 .…”

Electrodes modified with synthetic clay minerals: Evidence of direct electron transfer from structural iron sites in the clay lattice

“…The formation of CoO2 by oxidation of Co(OH)2 films, or by anodic polarization of Co electrodes has been directly demonstrated by Mt~ssbauer spectroscopy (Simmons et al 1976(Simmons et al , 1979. Figure 5 shows plots of the peak current versus the scan speed for peak I a of CMEs prepared with the two Co-smectites.…”

The electrochemical behaviour of cobalt in alkaline solutions part II. The potentiodynamic response of Co(OH)2 electrodes

“…The anodic formation of cobalt oxides has been studied by several authors since the works by Grube [1,2] The electrochemical behaviour of cobalt has been investigated by potentiostatic and galvanostatic techniques [3][4][5] and voltammetry [6][7][8][9], using different electrolytes such as alkaline aqueous solutions [10][11][12][13], neutral solution containing KH2PO4(+ NaOH) [14],borate solutions [14][15][16][17][18][19] and bi carbonate and borate solutions [20,21]. The different types of films produced at different potentials .were studied by ellipsometry [18,19,22] and Mössbauer spectros copy [23][24][25][26]. The equilibrium properties of the possible stoichiometric and nonstoichiometric film-forming species were also evaluated, including phase transition characteristics / [27][28][29][30][31][32][33][34][35][36].…”

“…There is also evidence of the occurrence of various sandwich-type structures in the different potential regions. In this sense the work of Sato and co-workers [15][16][17][18][19] is relevant, as well as the conclusions derived from Mössbauer spectroscopy [23][24][25][26] made in situ.…”