A Conductive film model for the kinetics of the lead anode in aqueous sulphuric acid
The kinetics and mechanism of the anodic process on lead in aqueous 35% H2SO4 (4.65 mol L-1) were studied. The lead anode is of interest as it is used in the form of the sponge lead negative plate in the lead-acid battery. Discharge of both planar and porous lead electrodes result in the electrocrystallisation of PbSO4. In the past, the processes occurring at the lead electrode have been shown to include: dissolution, precipitation, and passivation.
The techniques used were cyclic voltammetry, potential-step, and potential-ramp/potential-hold. The resultant transients were recorded both as analogue and digital waveforms. Investigations were carried out at a constant acid strength over a range of temperatures (-18° to 30°C), and in the presence of additives.
Significant variation was found in cyclic voltammetry with regard to the cathodic potential prehistory immediately prior to measurements. After extreme cathodic polarisation (below -1500 mV us SHE), voltammograms were recorded with relatively high anodic peak currents and charges. The extra charge was associated with the growth of a multilayer, and dissolution from the bare electrode. This multilayer is not reduced on subsequent moderate cathodic excursions (above -1000 mV). Hence for cyclic voltammograms in the absence of extreme cathodic polarisation one observes relatively small currents and anodic charges. This is due to transmission of Pb2+ ions through the multilayer under resistance control. Passivation occurs in both cases by precipitation of microcrystalline PbSO4 on the multilayer.
In contrast to cyclic voltammetry, potential-step transients were insensitive to cathodic polarisation and displayed current peaks due to growth of the surface film. One also observes a dissolution current from the bare electrode before it is covered by the growing film. The current first increases, passes through a maximum, and then decays with time. The increasing current was associated with growth of the multilayer, and the decaying current due to passivation by PbSO4.
A new model was successfully applied to the potential-step and potential sweep results. The model consists of several charge-transfer reactions: growth of a multilayer, dissolution from the bare electrode, and transmission of Pb2+ through the multilayer. The kinetics of the multilayer are pivotal to the other charge-transfer reactions. The inclusion of film transmission is the novel aspect of this model. Quantitative analysis of the model resulted in a set of optimised parameters that follow plausible variation with anodic potential. Both the potential step and potential sweep transients are adequately described by the model.
The effect of chloride added to the electrolyte was studied. The anodic and cathodic peaks in cyclic voltammetry are enhanced by this anion (100% more anodic charge at higher chloride concentrations) and the cathodic to anodic charge ratio is markedly increased. The general form of the potential-step transients are not modified by chloride. Evidence of a monolayer of PbCl2 was found (q = 500 µC cm-2) in both cyclic voltammetry and potential step experiments. The monolayer of PbCl2 must underlie the subsequent growth of the multilayer as the monolayer is first formed in potential-step experiments and first to be reduced in cyclic voltammetry at a small underpotential.
The effect of methyl orange on the lead anode was explored and found to enhance the charge capacity. Battery tests confirmed these observations, but methyl orange was destroyed by the oxidising PbO2 at the positive plate. Hence a derivative, designed to be insoluble in H2SO4, was synthesised. However, experiments on both planer and porous electrodes showed that the derivative, lauryl orange, C12H25NHC6H4NNH+C6H4SO3- (pKa = 4.06) was found to act as an inhibitor for the anodic reactions. In particular there was no contribution from multilayer growth in potential step experiments, and only dissolution (markedly diminished) and passivation were evident. Lauryl orange exhibited the required capabilities to bind to lead metal. The structure of lauryl orange was confirmed by n.m.r. and mass spectrometry.