Oxygen evolution on alpha-lead dioxide electrodes in methanesulfonic acid
Highlights
► Oxygen evolution on α-PbO2 in methanesulfonic acid medium. ► The reaction kinetics is slower in MSA than in sulphuric acid. ► The hydrated lead dioxide layer affects the reaction kinetics. ► A high MSA concentration leads to lower O2 evolution.
Introduction
A new type of redox flow battery, based on lead(II) dissolved in the electrolyte, was recently proposed by Pletcher et al. in a series of papers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. This soluble lead flow battery (SLFB) works with a single electrolyte made up of methanesulfonic acid (MSA, formula: CH3SO3H) which contains dissolved lead(II) ions in the form of Pb(CH3SO3)2. The reactor consists of two electrodes between which flows the electrolyte. Lead(II) is oxidised into lead dioxide at the positive electrode and reduced into lead at the negative during charge. During discharge, PbO2 and Pb turn back to Pb(II) in solution in MSA. The overall battery reaction is:
The lead(II) concentration in the electrolyte decreases when the battery is charged. In the same time, the protons concentration increases twice as fast.
The system differs from traditional lead acid batteries in that the electrolyte contains the active soluble lead(II) species, so that there is no solid phase reaction. The main advantage of this technology compared to other redox flow systems is that no membrane or separator is required since the electrolyte is the same for both electrodes. In addition, methanesulfonic acid is a less hazardous and more environmentally friendly electrolyte than other acids [11].
Oxygen evolution reaction (OER) taking place at the positive electrode during charge leads to a decrease in the faradic efficiency of PbO2 deposition, and hence leads to losses in energy efficiency and chemical imbalance at the two electrodes (excess of lead at the negative electrode) [6]. Moreover, it has been established [12] that uniform and well-adherent PbO2 layers are deposited from MSA electrolyte in low current density and low temperature conditions, whereas powdery or pitted PbO2 of poor adhesion are obtained using a high current density and/or high temperature, i.e. conditions favouring O2 evolution. It is highly possible that O2 evolution play a significant role in the loss of PbO2 in the form of small particles in the electrolyte during SLFB cycling encountered by Pletcher's team [10]. Therefore studying oxygen evolution reaction on lead dioxide material in methanesulfonic acid medium appeared to be worthwhile.
While there is quite a lot of the literature concerning oxygen evolution on PbO2 material in several electrolytes like H2SO4 [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], HClO4 [15], [22], H3PO4 [15], or CF3SO3H [21], no OER investigation on PbO2 was carried out in methanesulfonic acid, probably due to its recent application in lead acid batteries technology.
The general mechanism leading to O2 formation can be written as:H2O → OHads + H+ + e−OH → Oads + H+ + e−followed by the chemical recombination of oxygen:2Oads → O2
Pavlov et al. [17], [18] proposed that this mechanism take place on active centers PbO(OH)2 located in a hydrated lead dioxide layer at the PbO2/electrolyte interface according to:PbO(OH)2 + H2O → PbO*(OH)2…OH + H+ + e−PbO(OH)2…OH → PbO*(OH)2 + Oads + H+ + e−2Oads → O2
The kinetics of OER studied by steady-state polarization (current density vs. electrode potential), and represented by the slopes of the E. vs. log j plots (Tafel coefficient b), depends to a great extent on PbO2 structure (i.e. on the deposition conditions) [19], [24] and allotropic form (α or β) [14], [16]. The most commonly encountered b value is 120 mV [15], [16], [19], [22], which corresponds to a single electron transfer reaction with a α transfer coefficient of 0.5 (b = ln 10 × RT/αzF) at 25 °C. But values as high as 277 mV [24], or 344 mV [23] can be found.
The relative kinetics of the two electrochemical reactions is still not clear. Da Silva et al. [25] proposed that the primary water discharge (1) be the rate determining step (r.d.s.), Ho et al. [16] invoke the oxidation of adsorbed OH as r.d.s.
The present paper is dedicated to the study of O2 evolution on lead dioxide in methanesulfonic acid medium. Cyclic voltammetry was first used to calculate the kinetic parameters (Tafel coefficient and activation energy). Then electrochemical impedance spectroscopy was employed for further investigations on the reaction mechanisms. The lead dioxide allotropic form α was chosen because of its recognition by Pletcher et al. [7] to be the most convenient form for SLFB application due to its compact and well-adherent structure.
Section snippets
PbO2 deposition conditions
The disc electrodes on which lead dioxide was deposited were made up of a glassy carbon rod (diameter ∼0.3 cm, cross-section ∼0.071 cm2) embedded in Plexiglas tubes filled with epoxy resin. Before each deposition, the glassy carbon surface was pre-treated by anodic etching, subjecting the electrode to a constant anodic current (30 mA cm−2) in 1 M NaOH during 10 min, in order to improve PbO2 adhesion due to glassy carbon roughening [26].
α-PbO2 (250 μm thick) was electrodeposited onto glassy carbon
Voltammograms, Tafel and Arrhenius plots
A X-ray diffractogram and SEM images of a PbO2 coating deposited in the conditions described in Section 2.1. are shown in Fig. 1. The identification of the diffraction peaks was done using the PDF-2 database from ICDD. Most of the peaks can be identified to the α form of PbO2, while very few peaks corresponding to the other allotropic form of lead dioxide (β-PbO2) can be found. The fact that no significant peak exclusively associated to β-PbO2 can be observed strongly indicates that the large
Conclusions
Since evolution of oxygen is an undesirable reaction in the lead/methanesulfonic acid redox flow battery technology proposed by Pletcher et al., it was investigated on α-PbO2 electrodes in methanesulfonic acid at different concentrations, by cyclic voltammetry and electrochemical impedance spectroscopy.
Cyclic voltammetry revealed that the OER Tafel slope was rather constant with MSA concentration and temperature, with values ranging from 180 to 240 mV. Activation energies calculated from
Acknowledgements
The authors are grateful to ADEME (French environment and energy management agency) for its financial support of the thesis within which this study was carried out. They also thank David Brun-Buisson for the XRD analysis, and Julien Laurent for the SEM images.
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