Abstract
The charge-discharge characteristics and the aging mechanism of PbO2 layers in contact with sulfuric acid solutions of different concentrations (1.5–5.0 M) were studied by using combined cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) techniques. For this purpose, thick lead dioxide layers were electrodeposited on gold substrate from aqueous solutions of Pb(NO3)2 dissolved in nitric acid. Based on the electrochemical and the mass change responses, it was found that in more concentrated solutions of H2SO4, the main reduction reaction was the transformation of lead dioxide to lead sulfate. However, in less concentrated sulfuric acid media, the transformation of lead dioxide to lead(II) ion became the main reaction. These Pb2+ ions transformed into lead sulfate crystals later by a chemical reaction. Because the electrochemical oxidation of lead sulfate is less favourable in sulfuric acid medium of higher concentrations, thus, PbO2 layers cannot be tested by continuous cyclization, which is necessary to study their aging parameters. Therefore, a delay step before each cyclic voltammogram was applied while the non-conductive lead sulfate dissolves or alternatively, by applying a pre-oxidation step prior to each cyclic voltammetry experiment to produce electrochemically significant amount of lead dioxide which can be reduced during the following negative potential sweep.
Highlights
Lead dioxide layers and porous materials are widely used in industrial applications, e.g. common [1] and novel [2] lead-acid batteries, waste water treatment [5], and many other purposes [6,7,8]
The mass changes during the immersion of electrodeposited pure β-PbO2 layers in different concentrations of sulfuric acid solutions and their electrochemical behaviour in 1.5 M H2SO4 electrolyte using cyclic voltammetry experiments were investigated by Pech et al [13]
electrochemical quartz crystal microbalance (EQCM) experiments (Fig. 1a) were carried out on gold-coated quartz crystal in 0.1 M Pb(NO3)2 dissolved in 1.0 M HNO3 solution to study of the behaviour of the gold substrate and the electrodeposition process [18]
Summary
Lead dioxide layers and porous materials are widely used in industrial applications, e.g. common [1] and novel [2] (bipolar [3] and semi-bipolar [4]) lead-acid batteries, waste water treatment [5], and many other purposes [6,7,8]. Several studies concerning the electrochemical characterization of PbO2 films (charge-discharge characteristics, aging processes) have been carried out previously [9,10,11,12]. The mass changes during the immersion of electrodeposited pure β-PbO2 layers in different concentrations of sulfuric acid solutions and their electrochemical behaviour in 1.5 M H2SO4 electrolyte using cyclic voltammetry experiments were investigated by Pech et al [13]. It was concluded that a hydrated layer was formed on the surface of lead dioxide crystallites whose composition and thickness were independent from the concentration of the sulfuric acid solution. It was shown that both the crystalline and gelled PbO2 were transformed to lead sulfate during discharge. The gel zone that was consumed during the first cycle was regenerated from the remaining lead dioxide crystals reacting with the electrolyte according to Reaction 1.
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