Quantifying Electric Field Enhancement of Water Dissociation Rates in Bipolar Membranes

Abstract

This research investigated the effects of electric field strength and dielectric constant on rates of water dissociation into hydronium and hydroxide ions in bipolar membranes. Quantum chemistry simulations employing Møller–Plesset second order perturbation theory were used to calculate activation barriers for uncatalyzed water dissociation and for dissociation catalyzed by trimethylamine. Activation barriers for deprotonation of the trimethylammonium ion were also calculated as a function of the electric field strength and dielectric constant (εr). The activation barriers for water dissociation ranged from 63 kcal/mol in the absence of an electric field for εr = 20 to 52 kcal/mol at a field of 109 V/m for εr = 78. Hydrogen bonding decreased the activation barriers for water splitting but also resulted in neutralization of nascent hydronium and hydroxide ions in clusters containing five or more water molecules. Catalysis by trimethylamine reduced the activation barriers 42–61% compared to those without a catalyst. Deprotonation of the trimethylammonium ion was less dependent on the electric field strength than water dissociation and was nearly independent of the dielectric constant of the medium. The activation barriers for deprotonation of the trimethylammonium ion were greater than those for catalyzed water dissociation, indicating that the second step in the proposed water splitting mechanism is rate-limiting. For field values of 109 V/m, the enhancement in the rate of water dissociation was as high as 106 for the uncatalyzed process to 1010 when catalyzed by trimethylamine. Comparison of these results with those from Onsager’s approximate electrostatic model show that Onsager’s model significantly underpredicts field enhancement for εr = 78 and slightly overpredicts field enhancement for εr = 20.