Difference between surface electrochemistry of ruthenium and RuO₂electrodes

Abstract

A comparative study of the electrochemical behaviour of Ru and RuO 2 electrodes in solutions of 0?5M H 2 SO 4 , 1M HCl and 0?1M KOH has been performed with a view to elucidate in more detail the nature of the processes taking place on the surface of these electrodes within the range of potentials from 0?2 to 1?45 V (reversible hydrogen electrode, RHE). Cyclic voltammetry, chronopotentiometry and spectrophotometric analysis techniques have been employed. In accordance with thermodynamic data, it has been shown that amorphous layers of slightly soluble Ru(OH) 3 and RuO 2 .nH 2 O form on the surface of an active Ru electrode at 0?2,E,0?8 V and 0?8,E,1?1 V respectively, as a result of ruthenium anodic oxidation. Cathodic reduction of these layers proceeds reversibly within the indicated ranges of potential. It has been demonstrated that in the acidic medium, passivation of ruthenium electrode takes place at 1?1,E,1?35 V due to further oxidation of the ruthenium phase and formation of a more compact layer of anhydrous RuO 2 . The latter can be reduced only at potentials more negative than 0?2 V. In the case of alkaline medium, no passivation of the Ru electrode is observed within this particular range of potentials. At E.1?35 V, transpassive anodic dissolution of ruthenium electrode takes place in both acid and alkaline electrolytes concomitantly with oxygen evolution. The RuO 2 electrode demonstrates capacitor type voltammetric response at 0?4,E,1?4 V in both acid and alkaline medium. It has been proposed that such behaviour can be accounted for by a non-faradaic double layer process involving reversible adsorption/desorption of OH 2 ions on the surface of RuO 2 electrode as described by the following equation RuO 2 zH 2 O < Q ad RuO 2 :(OH À ) ad zH z where Q ad is the charge related to this process.

Corrosion of ruthenium dioxide based cathodes in alkaline medium caused by reverse currents

“…A large oxidation peak appears at about -1.05 V vs. SCE caused by re-oxidation of hydrogen gas trapped inside the nanoporous coating structure [6,23]. More important is, however, the oxidation peak at about +0.15 V vs. SCE indicating oxidation of Ru 4+ to Ru 6+ , which suggests formation of ruthenate ions, RuO4 2- [6,24,25]. The RuO4 2-formation is also observed in the first cycle of the reference sample, which is a consequence of that the cyclic voltammetry starts on the HER side.…”

“…4, it is possible that ruthenium species with higher oxidation states (Ru 7+ , Ru 8+ etc.) also were obtained during the corrosion process [14,25], but those peaks would be masked in the region for oxidation of Ni 2+ to Ni 3+ and the oxygen evolution reaction. The effect of HER, OER, and reverse currents on the crystal phases of RuO2 was studied through XRD and obtained diffractograms are presented in Fig.…”

Evaluation of Solubility of Ru in Acidic Solution

“…In the cyclic voltammogram of Ru, peaks were reported on the Ru electrode, as the hydrogen adsorption/desorption peaks around 0.2 V vs. RHE, the relatively reversible oxidation/reduction peaks around 0.4 V vs. RHE, and the irreversible oxidation/reduction peaks [4,6]. Figure 2 shows the cyclic voltammogram for a Ru-black/Au electrode at a 10 mV s −1 sweep rate in 1 M H 2 SO 4 at 25°C under N 2 .…”

“…In order to evaluate the stability and understand the degradation mechanism of the materials, their solubility or dissolution behavior should be understood. The surface electrochemistry of Ru and RuO 2 is reported, and slightly soluble species are amorphous Ru(OH) 3 and RuO 2 nH 2 O on Ru in the potential range from 0.2 to 0.8 V vs. RHE and 0.8 to 1.1 V vs. RHE, respectively [4]. However, few papers reported the solubility of Ru and other precious metals.…”

“…It has been reported that the reduction potential of the cyclic voltammogram decreases when the upper potential increases [4,6]. Therefore, the irreversibility of the surface oxide should be taken into account in order to evaluate the dissolution behavior of the Ru-containing electrode.…”