The effect of doping with Ti(IV) and Sn(IV) on oxygen reduction at hematite electrodes

We examined the reduction of oxygen at doped hematite (alpha -Fe2O3) electrodes to determine how the identity of the dopant affected the reaction and how changes in reactivity compared with changes in dopant concentration. Our results indicated that Sn(IV) and TI(IV) dopants have a similar effect on the open-circuit potential, cathodic transfer coefficient, and exchange current density, suggesting that any bandgap states associated with the introduction of dopants, and directly involved in the reduction of oxygen have similar energies. The greatest difference between the electrodes doped with Sn(IV) and Ti(IV) was in the apparent cathodic transfer coefficient. The cathodic transfer coefficients for the Sn-doped electrodes were slightly smaller than those for the Ti-doped, suggesting that the density of interface states is greater in the Sn-doped electrodes. In comparing electrodes with two different dopant concentrations, we found that the relative increase in reactivity was significantly less than the increase in dopant concentration. This may be due to the electrochemical creation of surface Fe(II) sites that catalyze the reduction of O-2 Other factors that may also contribute include bandgap states associated with the dopants and the fact that not all the dopants lead to creation of Fe(II) sites. (C) 2001 The Electrochemical Society. All rights reserved.