Effects of Carbon Nanotubes as an electrolyte additive on the electrochemical behaviour of GEL-VRLA battery Zhao Ruirui School of Chemistry and Environment South China Normal University Guangzhou China
1 introduction • Gelled electrolyte is the key factor affecting the performance of GEL-VRLA batteries; • the most commonly used gel electrolyte consists of fumed silica, sulfuric acid solution and some additives. • Fumed silica has good thixotropy, higher capacity than other gelling agent and reliability under cyclic or deep-discharge condition when used in gelled electrolyte, • shortcomings such as a short gelling time, a higher viscosity, an obvious increase of internal resistance and high cost, which limits extensive application of this technology.
Suitable additives are needed in order to overcome the shortcomings of the GEL-VRLA batteries. Glycerin, sodium silicate and phosphoric are most widely used materials. • This paper deals with the effect of multi-walled carbon nanotubes (MWCNs) as additive on the gelled electrolyte. The gelling property and electrochemical performances of the gelled electrolyte are used to evaluate the additive.
2 Experimental • The working electrode was pure lead, which was prepared by inserting a pure lead rod into a hard plastic tube and sealing with epoxy resin. One end of the electrode was welded with a copper wire. On the other end, a geometric area of 0.5cm2 was polished. • The gelled electrolyte was prepared by mixing fumed silica and 36wt. % H2SO4 solution (d=1.285g.ml-1). 0.02wt. % multi-walled carbon nanotubes (MWCNs) were added as additive. The mixture was dispersed in a homogenizer to form a colloidal solution; a high stirring rate about 3500rpm was needed. Electrochemical testing was performed after gelation of the colloidal solution.
3 Results and discussion3.1 Gelling property study • The results show that the addition of WMTNs can improve the gel strength, however, the viscosity increased and the liquidity essentially constant as the additive was introduced.
3.2 Cyclic voltammetry (CV) • Fig.1 shows the 50th CV plots of Pb electrode in different electrolytes. Two peaks appear in the plot, the oxidation peak to the Pb to PbSO4, and the reduction one to the PbSO4 to Pb. • The peak current of the lead electrode in the electrolyte with WMTNs is bigger can its counterpart. That means the additive can improve the charge/discharge capacity. The additive carbon nanotubes which have good conductivity must be the main cause. Fig.1 CV plots of lead electrode in gelled electrolytes.
3.3 PbO2 formed on lead alloy in different electrolytes • Fig.2 shows the Nyquist plots of the impedance, which was performed at a potential of 1.3V over the range 100MHz to 0.01Hz with AC amplitude of 5mV, the fitted value are listed in table 1. Tab.1 parameters of EIS devised from fig.2 Fig.2 Nyquist plots of lead electrode in gelled electrolytes at 1.3v.
The semicircular radius of the electrode without additive is much larger than another, what demonstrates that the additive can make the growth of PbO2 difficult. As can be seen in table 1, the value of Rct for the reaction in the electrolyte without additive is larger than another. This suggests that MWNTs can suppress the growth of PbO2 on lead electrode.
3.4 PbO formed on lead alloy in different electrolytes • the anodic Pb(Ⅱ) film is the major component at the potential of 0.9. As shown in Fig.3. The electrochemical impedance plots of the pure Pb electrode in different electrolytes are similar. The plots show a semicircular region at high frequency and a linear region at low frequency, which are associated with a charge transfer step and a diffusion-controlled step, respectively. The semicircular radius of the electrode with additive is much larger than another, what demonstrates that the additive can promote the growth of PbO. Fig.3 Nyquist plots of lead electrode in gelled electrolyte at 0.9v.
3.5 Oxygen evolution and hydrogen evolution study • Fig.5 shows the CV plots of the electrode, the potential of hydrogen evolution negative shift, which means the addition of Carbon nanotubes can inhibit the hydrogen evolution. The plots of electrochemical impedance measurements were shown in Fig.4, the coincident result with CV study can be obtained. However, as can be seen in Fig.6, the over potential of oxygen evolution on the Pb in electrolyte without additive is lower than its counterpart. Fig.5 CV plots of lead electrode in gelled electrolytes.
Fig.4 Nyquist plots of lead electrode in gelled electrolyte at -1.5v. Fig.6 Oxygen evolution plots on lead electrode
4. Conclusions • The addition of multi-walled carbon nanotubes can inhibit the growth of PbO2 and the hydrogen evolution reaction on the surface of Pb alloys electrodes. • however, the formation of Pb(Ⅱ) oxide film was improved and the over potential of the oxygen evolution reaction was depressed.