Cu x S (x → 2) thin films were obtained by sulfidization of copper thin films previously obtained by spin-coating from a dichloromethane solution of [Cu(II)(2-ethyl hexanoate)2(H2O)2] deposited on ITO substrate, irradiated with UV light and electrochemically reduced. Through cyclic voltammetry experiences performed in a 0.05 M Na2B4O7 buffer solution containing 5 mM Na2S, the electroformation mechanism of Cu x S phase is controlled by a first electron transfer, obtaining an initial formation of Cu(HS)ads that evolves to a Cu x S phase. Potentiostatic current transient recorded in the potential range of −0.8 V ≤ E ≤ −0.7 V showed that the nucleation and growth mechanism of the Cu x S phase obeys a two-dimensional instantaneous process with diffusional and charge-transfer contributions. AFM analysis of the deposits shows that Cu x S phase is preferentially deposited in the valleys left by ITO particles. The average size of Cu x S particles is close to 20 nm. Cyclic voltammetry results, electromotive force determination in the Cu/ /Cu x S galvanic cell, EDAX, and UV analysis demonstrate that the stoichiometric factor x in Cu x S is close to 2. The electro-obtained Cu x S phase was unstable and evolved to other nonstoichiometric compounds at open circuit. The processes responsible for the instability were the own oxidation of Cu x S phase and the water reduction that takes place over Cu x S and bare ITO particles. The last process was studied by electrochemical impedance spectroscopy. Photoelectrochemical measurements in the stability potential range of the Cu x S phase shows that the electro-obtained phase presents a p-type conductivity.

During the electrochemical oxidation of Prussian blue (PB) to Prussian yellow (PY), an electrocatalytic oxygen production proceeds at the electrode when aqueous electrolyte solutions are used. The formed oxygen is scavenged by the PY, probably by absorption, and it is consumed during the electrochemical reduction of PY to PB by a heterogeneous chemical reaction of PB with oxygen to PY and hydrogen peroxide. Because of this catalytic regeneration of PY, it is impossible to determine the amount of low-spin iron by chronocoulometry using a potential program in which PB is first oxidized to PY and then the charge is measured to reduce PY to PB. The latter charge is biased by the electrocatalytic PY regeneration.

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