In the manufacture of negative plates for the lead-acid battery it is customary to add to the lead oxide-sulphuric acid paste forming the active material approximately 0.3~o of a derivative of lignosulphonic acid. Such additives are generally termed expanders. It is claimed that this produces an improvement in cycle life, and gives a better high discharge rate performance, particularly at low temperatures 1-4. The mechanism by which the expander exerts its beneficial effect has been the subject of several studies I 1~, but there does not appear to be complete agreement on the mode of action at the present time. This may be due partly to its functioning in more than one way, and because of this it may not be possible to analyse expander action fully from experiments on conventional battery plates. The majority of early workers concluded that the most important function of the expander was to modify the crystalline structure of the lead sulphate formed on discharge. More recently attention has been directed at the actual oxidation of lead to lead sulphate, and the possibility that the expander may influence the kinetics of this process. Both Sharpe 9, and Niklas and Jakobljevich 1~ have reported that an expander enhances the height of the peak obtained when a lead electrode is swept through the Pb/PbSO4 potential to passivation. Archdale and Harrison have also made the interesting suggestion that the expander may preferentially suppress the solid-state oxidation of lead, in favour of the dissolution-precipitation process ~ z. It was mainly to test the latter claim that the present investigation was carried out. Linear sweep voltammetry studies were performed at a stationary lead electrode in 5-M sulphuric acid with the addition of several commercial expanders. Differential capacitance measurements were also made in sodium sulphate solutions of varying concentrations to assess the extent to which these materials are adsorbed on a lead surface. Sharpe 9' lO and Yamporskaya and Kabanov 6 have reported strong adsorption of expander materials upon metallic lead, and there is general agreement that the expander function involves adsorption on the plate material.
EXPERIMENTAL
Apparatus
Electrolytes were prepared from AnalaR grade sodium sulphate or sulphura: acid and bidistilled deionised water. For the differential capacitance measurements