Solubility and Stability of Silver Oxides in Alkaline Electrolytes

Abstract

The solubility of Ag20 and AgO in alkaline electrolytes has been studied with a polarographic technique, using a rotating platinum electrode. Only a, monovalent (and no divalent) silver species could be detected in solutions which had been in intimate contact with AgO powder over prolonged periods of time.Quantitative measurements of the solubility of Ag~O in KOH solutions ranging from 1-14 moles per liter, were carried out utilizing a potentiometric titration method. The solubility has a maximum at about 6N KOH where it reaches a value of 4.8-10-4N.The rate of the decomposition reaction 2AgO~ Ag~O + 1/2 O2 in alkaline electrolytes was investigated with a very sensitive microvolumetric method. The reaction rate increases with increasing hydroxyl ion concentration and is sensitive to day-light. The decomposition appears to proceed at the solid-liquid interface. Any divalent silver dissolving in minute amounts into the electrolyte is decomposed rapidly due to its instability. The decomposition proceeds at a rate of about 16% in 1 year at 30~ and 49% in 1 year at 45~ This reaction can contribute significantly to the self-discharge of AgO-Zn batteries.Anodic oxidation of silver in alkaline solutions is a two-step process. First, a layer of monovalent Ag20 is produced on the metal. Subsequently, when this layer has reached a critical thickness and mass transport through the layer becomes slow, Ag20 is oxidized to AgO. Further anodization leads to evolution of oxygen. Simultaneously, the oxidation of metallic silver continues at a very low rate. There is also evidence of the formation of highly unstable "higher oxides" than AgO during prolonged anodization.The standard potential of the Ag/Ag~O couple in alkaline solutions is 0.342 v (1) and the potential of the Ag~O/AgO couple, 0.604 v (2). If an electrode contains both metallic silver and AgO, a local action mechanism must proceed (3, 4) with a driving force of 0.262 v or --6.05 kcal, according to 2Ag + 2(OH)-~-~--~ Ag~O + H20 + 2eEo~ 0.342with the first reaction proceeding in the forward direction, and the second reaction in the backward direction, resulting in Ag + AgO ~ Ag20 AG = --6.05 kcal This local action mechanism should be a fast process, because the individual component electrode reactions are known to have high exchange currents. Measurements of the double layer capacity and the interface resistance during anodic formation of Ag20 films (3, 5, 6) give an insight into the mechanism of oxidation. The double layer capacity decreases during build-up of the Ag~O layer, apparently due to increased separation of the charge layers in the solution and on the metal. At the same time, the interface resistance increases sharply, indicating restriction of mass transfer. The resistance decreases again when the formation of AgO begins. This decrease in resistance must be correlated with the breakdown of the Ag~O layer, and with the high electric conductivity of the divalent oxide. The conductivity is in the order of 7.10 -~ ohm -1 cm -1 and increases with increasing tempe...

Reaction Pathway of the Hydrothermal Synthesis of AgCuO2 from In Situ Time-Resolved X-ray Diffraction

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Investigating Degradation Modes in Zn‐AgO Aqueous Batteries with In Situ X‐Ray Micro Computed Tomography

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“…[ 17,37–39 ] However, as shown in Equations (3,4), OER can facilitate AgO decomposition, resulting in capacity fade and self‐discharge in Zn‐AgO batteries. [ 40,41 ] …”

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