Ruizhen Li School of Chemistry and Environment South China Normal University Guangzhou China Study on Lead Based Rare Earth Alloys for Positive Grids of Power and Energy Storage Lead-acid Battery

Contents Introduction 1 Experimental 2 Results and discussion 3 Conclusion 4

1 Introduction Possessing of the potentials of development, solar energy has wide prospects, as the key part of the solar electric generation system, storage energy battery becomes the restrictive factors. Meanwhile, acted as a green intelligent and efficiency transportation vehicle, electric bikes encounter opportunity and challenge too. Valve regulated lead-acid battery was the first selection for solar system and E-bikes for many years, service life is the most important which restrict the development of the two fields

 There are many literatures on the passive film, but the problem still exists. Premature capacity loss (PCL) of the traditional lead-calcium alloy was very severe and the passive film had bad conductivity.The rare earth elements were often used as additives to lead alloys in order to get better properties for lead-acid batteries, cerium was added to improve the hydrogen evolution performance, corrosion resistance would be increased too, lanthanum and cerium could inhibit the anodic film formation in deep discharge at the potential of 0.9v.  In this paper, ratio optimization on the most widely used alloy Pb-Ca- Sn-Al was made, argentine and mixed rare earth of lanthanum and cerium were introduced to the alloy, and a new practical grid alloy Pb-Ca- Sn-Al-Ag-La-Ce was developed.

2 Experimental  Lead-calcium tin aluminum argentine alloys (with 0.07, 1.2, 0.05, 0.1wt.%) were processed in the laboratory by melting weighed mixtures of pure materials, mixed rare earth lanthanum and cerium was added with mass ratios of 0, 0.01, 0.15, 0.5wt.%, respectively. In this paper, 1#, 2#, 3#, 4# were used to represent these alloys, respectively.  Electrochemical tests were performed in a three-electrode cell, the counter electrode and reference electrode were a platinum plate and Hg/Hg 2 SO 4 electrode (1.28g.cm -3 H 2 SO 4 solution, E= vs. SHE), respectively.

3.1 Alternating current voltammetry (ACV) Fig1 Z'vs.E plots of the anodic films formed on electrodes at 0.9Vfor 1h (v= 1mV.s-1,f=1000Hz) Alloys with mixed rare earth elements have lower resistance at the Pb( Ⅱ ) film reduced potential about 0.75V, the same phenomenon appeared at the potential of -0.75V, where is the electric resistance peak of PbSO 4. It can be concluded that the addition of the mixed rare earth lanthanum and cerium can hinder the growth of the anodic Pb( Ⅱ ) film, the deep cyclic life will be improved.

Fig.2 The open circuit decay curves of the electrodes 3.2 Open-circuit potential (OCP) The depression curves of the alloys after oxidation at 0.9V for 1h are shown in Fig.2. A platform EF appears at -0.5V, which corresponding with the transformation time(TEF) of Pb( Ⅱ ) to Pb, the length of TEF represent the amount of Pb( Ⅱ ). The electrode without additive has the longest platform, the amount of Pb( Ⅱ ) decrease as the content of rare earth increase, This indicated that the addition of rare earth can inhibit the growth of Pb( Ⅱ ) film. This result is consistent with ACV.

3.3 Chronoamperometry (CA) Fig.3 Current-time curves different electrodes at(a 1.5V, b 1.6V) Fig.3 provides the plots of current vs. time at 1.3V, peak a appeared in the curve corresponding with the formation of PbO 2. It can be seen from the curve that the peaks of the electrodes with lanthanum and cerium are lower than their counterpart. It seems that adding this material can inhibit the formation process, however, when the additive is higher than 0.5wt. %, this phenomenon disappears. In this case, proper amount of mixed rare earth lanthanum and cerium can promote the anti-corrosion performance of the grid.

3.4 hydrogen evolution study Fig.4 Cathodic polarization curves alloy electrodes in 1.28 g.cm -3 H 2 SO 4 solution (v=5mV.s-1) Table 1 kinetic parameters of the hydrogen evolution reaction on different electrodes electrode 1# 2# 3# 4# a b

 The rate of hydrogen evolution was studied by LSV and the results are shown in Fig.4. The kinetic parameters of the reaction on the electrodes are presented in table 1. It can be seen that a value is different, which represents the over-potential of the hydrogen evolution reaction, the value increased with the amount of the mixed La and Ce. This indicates that the addition in the alloys inhibits hydrogen evolution reaction.

3.5 Oxygen evolution study Fig.5 Electrochemical impedance spectra of the oxygen evolution reaction on electrodes at （ a 1.4 b 1.6V ） Fig.6 The equivalent circuit of Fig.5 The plots for these electrodes are similar and exhibit a semicircular part at high frequency that indicates control by electron transfer. The semicircular radius of the electrode without additive is much larger than other electrodes, which demonstrate that the additive of mixed rare earth elements can inhibit the oxygen evolution. This suggests that the oxygen evolution reaction is influenced by the addition of mixed La and Ce.

3.6 SEM micrographs of the corrosion test 1# 1000X 2# 1000X 3# 1000X 4# 1000X

1# 5000X 2# 5000X 3# 5000X 4# 5000X Fig.7 Cross-sectional views of the corrosion layer on electrodes

 The SEM micrographs reveal the differences in the morphology of the corrosion layers after corrosion test of the electrodes are shown in Fig.7. A loose and porous corrosion layer is formed on the electrodes with mixed La and Ce. This is beneficial to the charge/discharge of the battery, for the active materials can reach the grid more easily, utilization ratio of the active materials can be increased. The deep charge/discharge performance of the battery will be improved by the addition of mixed La and Ce.

4. Conclusion 1 Mixed rare earth elements La and Ce can inhibit the growth of anodic Pb( Ⅱ ) film at 0.9V, and decrease the resistance of the oxide film, the deep cyclic life will be improved. 2 Both hydrogen and oxygen evolution performance can be improved by this addition. The hydrogen evolution over- potential and oxygen evolution over- potential are higher and the charge/discharge property of the battery will be better. 3 Proper amount of mixed La and Ce can inhibit the growth of lead dioxide, what is beneficial to the anti-corrosion of the alloy. However, the content must lower than 0.5wt. %. The optimal content is 0.15wt. %. 4 The corrosion products of the new alloy Pb-Ca-Sn-Al-Ag-La-Ce are loose and porous that the active material can contact to the grid surface easily.