Water adsorption on lead dioxide from ab initio molecular dynamics simulations

The electrochemically active lead dioxide (β-PbO₂) contains the hydrogen (H) species inside the bulk and on the surface. The loss of the surface H species is proposed to be one of the factors in lead-acid battery failure. In this study, water adsorption on β-PbO₂ has been investigated using theoretical approaches to reveal the chemical forms of the surface H species and identify a probable cause of H loss mechanisms. For the single water-β-PbO₂, density functional theory (DFT) calculations present intact water molecular adsorption on β-PbO₂ (100) and dissociative water adsorption on β-PbO₂ (110), (101), and (001) surfaces. The geometric distances and the number of hydrogen bonds contribute to the adsorption energy reduction of single water adsorption. For the liquid water-β-PbO₂ slab models, DFT-based molecular dynamics simulations observe that the surface lead sites are fully occupied by a hydroxyl group or intact water molecule, and some of the surface oxygens are protonated at 300 K. On the β-PbO₂ (110) termination, dissociative water adsorption and intact molecular water adsorption occur competitively, leading to about 50% dissociation of adsorbed water molecules. On the β-PbO₂ (100), (101), and (001) terminations, the water molecules adsorb preferably in the dissociative form. The surface dependence of water dissociation is explored in terms of hydrogen bonding interactions relevant to adsorbed aqueous species. It is indicated through the Wulff crystal shape that the increase in the β-PbO₂ crystallite size may be one of the H loss mechanisms associated with the electrochemically inactive β-PbO₂.