Hydrogen titanates, H₂Ti₃O₇ (HTO) are highly versatile, non-toxic and corrosion-resistant materials that have been promising for electrochemical energy storage and conversion devices. One unresolved issue with this class of materials for energy storage is the low Coulombic efficiency (CE) in strong acid electrolytes. In particular, a complete loss of redox signature is observed when HTO is cycled in H₂SO₄ electrolyte. On the other hand, an increase in anodic charge storage capacity and CE is observed when H₂SO₄ electrolyte is replaced with H₃PO₄. SEM of electrodes reveals higher rates of surface pitting of the HTO rods in H₂SO₄, compared to H₃PO₄, suggesting that the loss of capacity results from irreversible structural transformation or dissolution of HTO. However, the dissolution mechanism, and the effect of the electrolytes and the oxidation state of Ti such as Ti(III) and Ti(IV) on the dissolution are not clear. In this presentation, we describe the ReaxFF reactive molecular dynamics (MD) simulation of HTO in H₂SO₄ and H₃PO₄ solutions to understand the role of proton surface reactivity and insertion in HTO coupled to dissolution of Ti(III). The ReaxFF method that combines concepts of bond-order based potentials with a polarizable charge distribution is a highly transferable simulation method for atomistic scale simulations on chemical reactions at the nanosecond and nanometer scale. Using ReaxFF MD simulations, we propose that higher levels of solvation of surface oxygens lead to Ti(III) dissolution – a process that is mitigated in H₃PO₄ due to the higher pK_a and the adsorption and concentration of H_xPO₄ at or near the HTO surface.

Yun Kyung Shin

Appearances