Origin of the “Excursion Peak” during cycling voltammetry of Pb–Sn alloys
Introduction
Lead-acid battery (LAB) was invented in the first half of the XIX century by Gaston Plante making it the oldest rechargeable systems of an electrochemical storage of energy. Despite this fact, LAB is still a significant part of the battery market. It is caused by their advantages over newer types of batteries, such as reliability and low cost. Additionally, while this type of battery has been in widespread use for a long time it is still under further development, constantly improving its properties including its biggest weakness: comparably low specific energy. Moreover, despite over 150 years of research and common usage there are still some unexplained areas in its electrochemical behavior.
One of the phenomena in the lead electrochemistry is the so-called anodic excursion peak [1], [2], [3], [4], [5], [6], [7]. It is a small anodic peak or peaks appearing on cyclic voltammetry curves during the reduction of PbO2 formed by the oxidation of a lead electrode. When such electrode is polarized toward more negative potentials, an oxidation peak (or peaks) in the neighborhood of the reduction peak of PbO2 is observed. It is an uncommon observation that the polarization of an electrode towards negative potentials causes an anodic current to appear. The shape, position and number of the excursion peaks is highly dependant on experimental conditions, such as temperature or acid concentration. In different conditions some peaks may be overshadowed by a bigger PbO2 reduction peak and even disappear completely. This leads to the total number of peaks to be not agreed upon, with proposals ranging from one broad excursion peak [8] to four separate peaks [9]. As a result, this leads to great difficulties in understanding the mechanism behind the generation of the peak. Nevertheless, several ways of explaining this mechanism were proposed.
The earliest presented mechanism of the excursion peaks appearance by Panesar described formation of tetravalent lead compounds in 5 M H2SO4 [10]. In 1976 Sunderland proposed that the reaction responsible for the peak is the oxidation of water to O2 and H+ by Pb3+ ions. This Pb3+ ions are in turn generated during the first step of the PbO2 surface reduction [1], [2].
Deutcher et al. [3] have suggested that during the reduction of PbO2 there are cracks appearing in the corrosion layer. It is the result of significantly higher molar volume of PbSO4 compared to PbO2 (48 cm3/mol for PbSO4 and 25 cm3/mol for β-PbO2). It means that the reduction of PbO2 puts a considerable strain on the corrosion layer due to its swelling and results in appearance of fractures. These cracks can be deep enough to reveal a metallic lead under the corrosion layer with electrode still at oxidative potentials for Pb → Pb2+ reaction. Due to the applied potential, the exposed metal undergoes oxidation in H2SO4 to PbSO4, causing the appearance of the anodic current and consequently, the excursion peak. This mechanism was supported by many researchers in later works [4], [5], [6], [7].
Another explanation proposed by Zhang et al. in 2011 [9] is that the anodic excursion peaks are the consequence of the oxidation of PbOn intermediate, where 1 < n < 2. In this work four different excursion peaks were recognized and attributed to four different PbOn intermediates (with n values of 1.37, 1.44, 1.55, 1.57). PbOn was generated during an incomplete oxidation of PbO to PbO2, owing to the inhibition caused by a compact PbSO4 layer. This sulfate layer was decreasing the electro-migration rate of OH− ions. Presented approach suggests that the size of the excursion peaks can be used to characterize corrosion resistance of a lead material [9].
As described above there is currently no unanimity regarding the exact mechanism of the excursion peak appearance. The matter is further complicated by different behavior of this peak presented in different studies caused by varying experimental conditions. Having observed the conditions of the experiment and their influence on the behavior of the peak one can make some presumptions about the underlying mechanism of its formation. Pavlov et al. examined the influence of potential scan rate and acid concentration on the Pb/PbO2/PbSO4 corrosion layer structure [11]. One of the earlier studies carried by Czerwiński et al. investigated the excursion peak for pure lead in a sulfuric acid electrolyte with different concentrations and at different temperatures [12]. This paper followed earlier works on pure lead electrodes behavior in sulfuric acid [13], [14]. Paleska et al. examined this peak for different lead alloys with a phosphoric acid additive to a standard sulfuric acid electrolyte [15]. Darowicki and Andrearczyk studied the excursion peak phenomenon with the use of atomic force microscopy and electrochemical impedance spectroscopy [16].
In this work the behavior of the excursion peak for lead–tin alloys with different tin contents was examined. Alloys with tin were chosen because of their common use as a corrosion-reducing additive in commercial batteries. Moreover, the mechanism of tin influence on lead corrosion is well described in the literature [17], [18], [19], [20]. The lead–tin has been also used for modification of conductive porous carbon (CPC) grids applied in new high energetic carbon lead acid battery (CLAB) [21]. The behavior of these alloys was tested for different acid concentrations and sweep rates. Additionally, changes in morphology of the corrosion layer during the excursion peak formation were examined using the Scanning Electron Microscope (SEM).
Section snippets
Experimental
The measurements were conducted using a three-electrode vessel. The counter electrode was a 0.5 mm lead sheet. The reference electrode was a Hg/Hg2SO4 electrode with 1 M H2SO4 as an electrolyte. All potentials in this paper are given with respect to this electrode potential (660 mV vs. SHE). The working electrode was a disc electrode made from a lead–tin alloy. It was made from lead and tin in desired proportions and melted twice. The alloy was casted in a form of a cylinder and held in 200 °C for 2
Results and discussion
One of the inadequacies of the excursion peak mechanism that assumes corrosion layer cracking is that the reduction of PbO2 to PbSO4 is supposed to be occurring before the excursion peak. The reason for this is that only after the reduction has started, the generated stress in the corrosion layer can cause fractures to appear and the metallic lead undergoes oxidation. However, it was observed that in some conditions the anodic peak is formed before the main reduction peak of PbO2 [8], [9], [12]
Conclusions
In the presented work the behavior of the excursion peak in different experimental conditions for lead–tin alloys was investigated. The appearance of cracks in corrosion layers during the excursion peak formation has been proven by SEM images. These fractures are believed to reveal the metallic lead which undergoes oxidation, causing a generation of the excursion peak. The behavior in different acid concentrations was found to be consistent with this mechanism. The presented influence of the
Conflict of interest
There is no conflict of interest.
Acknowledgements
This work was financially supported by Industrial Chemistry Research Institute through projects nr 841401, 841305 and 841331.
References (24)
J. Electroanal. Chem.
(1976)- et al.
J. Electroanal. Chem.
(1981) - et al.
Electrochim. Acta
(1986) - et al.
Electrochim. Acta
(1991) - et al.
Electrochim. Acta
(1991) - et al.
Electrochim. Acta
(1983) - et al.
J. Power Sources
(2011) - et al.
J. Power Sources
(2004) - et al.
J. Power Sources
(1997) - et al.
J. Power Sources
(2003)