An RDE Investigation of NaOH Effect on Electrosynthesis of Lead Dioxide in Alkaline Solution

Abstract

An electrochemical investigation of NaOH concentration on electrosynthesis of lead dioxide in alkaline solutions was performed using rotating disk electrodes (RDE). The results show that Pb(II) does not modify the evolution of oxygen in alkaline solutions, and the reaction taking place at 0.84 V SHE is under the mixed control of ionic transport and charge transfer. The Koutechy-Levich equation was used to calculate the value of diffusion coefficient D and apparent heterogeneous rate constant k of the PbO 2 electrodeposition process at 0.84 V SHE . The results indicate that NaOH has a negative influence on the diffusion of Pb(II) within a NaOH concentration range of 2.5 ∼ 4.5 M. In addition, NaOH has a positive influence on the apparent heterogeneous rate constant of Pb(II) oxidation reactions within a NaOH concentration range of 2.5 ∼ 3 M, but a negative influence within a NaOH concentration range of 3 ∼ 4.5 M. XRD and SEM were employed to investigate the effects of NaOH concentration on phase composition and surface microstructures of deposits. The results show that PbO 2 synthesized in an alkaline solution consists of pure α phase. The deposits are all composed of rounded nanocrystallites, except for the deposit synthesized in a solution of 3 M NaOH, which exhibited rod-like crystallite. Lead dioxide (PbO 2 ) has attracted considerable attention, owing to its low electrical resistivity, low cost, ease of preparation, good chemical stability in acid media, high overpotential for oxygen evolution reactions (OER), and relatively large surface area.1-4 Thus, lead dioxide has already been used in lead batteries, 5-7 ozone generation, 8,9 waste water treatment, 10-12 analytical sensors, 13 and the electrowinning process.14 It is well known that PbO 2 has two phases: the α phase and β phase. Electrodeposition is a traditional way to prepare these two phases of PbO 2 . 15 The conditions (mainly composition and temperature) of the synthesizing bath determine the phase of deposits. 16 To the best of our knowledge, α-PbO 2 is less studied than β-PbO 2 due to its lower electrochemical activity and electrical conductivity.17 Despite the shortcomings mentioned above, α-PbO 2 is chemically more stable than β-PbO 2 ; consequently, it promotes longer life cycles of lead-acid batteries.18 Besides, it is beneficial to ozone production, 19 presents the same performance of β-PbO 2 as a pH sensor 20 and is the first step in the electrochemical fabrication of metallic lead nanowires (NWs). 21 In addition, as an interlayer of the electrode, the binding force of β-PbO 2 and its underlayer would be improved by α-PbO 2 .22 Pure α-PbO 2 can be deposited from an acetic acid lead bath 23 or a methanesulfonic acid lead bath 24 at suitable conditions. However, a stress-free α-PbO 2 can be obtained by electrodeposition from an alkaline lead bath. 25,26 Cyclic voltammetry (CV) technology is a general and powerful method to study reactions in aqueous solutions, surface adsorption and deposition. 27,28 It can be used to investigate the kinetics ...