Search

Bipolar ion-exchange membranes (BPM) have a long history and many uses. Still, the number of applications continues to grow every year, as does the interest in a better understanding of the mechanisms of ion transport and water splitting. We reconsider the existing theoretical background for the BPMs and propose a one-dimensional stationary model. For the first time, the Nernst-Planck-Poisson equations are coupled with the equations, which locally describe the water splitting kinetics. The catalytic effects exerted by the functional groups and the catalyst (if present), the protonation-deprotonation reactions of these groups as well as the effect of strong electric field (the second Wien effect) are taken into account when describing the reverse-bias mode. The effective rate constant of water splitting occurring in the absence of electric field is determined from the acid dissociation constants (K a). The concentration profiles, the thickness of the reaction zone and space charge region (SCR) are simulated and analyzed. The results of numerical computation are compared with the known analytical estimations. Thus, the basic assumptions of the theory of BPMs are examined, and the conditions under which these assumptions are valid or not are found. Current-voltage curves and partial currents are calculated and compared with experimental data. The occurrence of two limiting currents at pK a close to 7 is predicted due to current-induced membrane discharge. According to simulations, any increase in the SCR thickness leads to a decrease of the water spitting rate at a fixed voltage due to a decrease in the second Wien effect. A very thin (about 4 nm) catalytic layer within the bipolar junction is sufficient to obtain 100 mA cm−2 current density at 1 V even when the anion-exchange and cation-exchange membrane layers do not contain catalytically active functional groups. Image 1 • 1D stationary model is developed on the basis of the Nernst-Planck-Poisson equations. • The catalytic effects exerted by functional groups and catalyst are described. • The occurrence of two limiting currents at pK a close to 7 is predicted. • The second limiting current is due to current-induced membrane discharge. • Any increase in the depletion thickness leads to a decrease in water spitting rate. [ABSTRACT FROM AUTHOR]