does phosphoric acid show the redusing property.if possible then give an example
Answers
Explanation:
3.12.1.1 Phosphoric Acid, H3PO4
Phosphoric acid has been used as additive to lead–acid batteries since 1920 and a number of patents have been issued claiming that H3PO4 improves the performance of batteries of different designs and applications [21]. Voss [22] and Meissner [23] have published comprehensive reviews of the effect of phosphoric acid and phosphate salts on the performance of lead–acid batteries. In general, addition of phosphoric acid to battery electrolyte has a diverse effect, for example, it prolongs battery cycle life by reducing the shedding and irreversible sulfation of the positive active material but reduces the capacity of the battery. Many controversial effects of phosphoric acid have been reported in the literature. For this reason, much efforts have been put forth to elucidate the mechanism of action of phosphoric acid. Linear sweep voltammetry technique on smooth lead and lead alloy electrodes is mainly used to examine the influence of H3PO4 addition on the electrochemical kinetics of PbO2/PbSO4 reactions, as well as on oxygen evolution. It is established that, in the presence of phosphoric acid, the equilibrium potential of the positive electrode shifts to more positive values [24,25]. Phosphate ions reversibly absorb in the PbO2 during charge and modify the nucleation and crystal growth of PbO2. During discharge, H3PO4 is released. It has been found that phosphoric acid is incorporated in the corrosion layer, too [26,27]. Experimental data evidences that H3PO4 has a beneficial effect on the performance of positive battery plates, when added in amounts of up to 0.8 wt%. Above this concentration, phosphoric acid has an adverse effect [27].
Generation of Pb3(PO4)2 as an intermediate product of the process of PbO2 formation at low H3PO4 concentrations has been established by Bullock [24,28]. Phosphoric acid increases the hydrogen and oxygen overpotentials [29–32]. Addition of H3PO4 to sulfuric acid electrolyte has been found to prevent the early capacity decline (premature capacity loss) of positive plates with Pb–Ca grids on deep discharge cycling [33–35]. It has been established that addition of H3PO4 to sulfuric acid electrolyte prevents passivation of positive plates with Sb-free grids on deep discharge cycling and facilitates formation of α-PbO2 modification as well as of fine PbSO4 crystals during discharge [36,37]. Most probably, the latter two effects are responsible for the decline in capacity of positive plates when phosphoric acid is added [38].
It has been found that H3PO4 prevents capacity decay of the positive electrode during cycling of gelled lead–acid batteries for electric vehicle applications [23]. Addition of phosphoric acid to VRLAB electrolyte yields stable capacity performance of these batteries in different solar power systems [39].
When phosphoric acid is added to the electrolyte in combination with H3BO3, SnSO4, and picric acid, corrosion of the positive grids is notably slowed down [40]. For standard automotive batteries, Torcheux et al. have patented an electrolyte additive formula containing 2.2% phosphoric acid and 4% colloidal silica [41]. The use of phosphoric acid in combination with colloidal silica prevents positive active mass softening, decreases acid stratification on cycling at high discharge rates, and improves significantly the cycle life performance of lead–acid batteries [41].