The Effect of Doping with Ti(IV) and Sn(IV) on Oxygen Reduction at Hematite (Fe2O3) Electrodes

We examined the reduction of oxygen at electrodes made with doped hematite (i.e., hematite containing a small percentage of impurities). Doped hematite is expected to have additional Fe(II) sites. Fe(II) sites in Fe(III)-containing iron oxides typically increase the reducing capability of the oxides and for this reason can have a significant impact on the biogeochemistry of natural systems and on the continued corrosion of iron structures. Iron oxides found in natural and antropogenic systems are likely to be impure and therefore, in essence, doped. Thus, it is important to examine the details of how the introduction of dopants affects the reactivity of Fe(III)- containing oxides.

We were interested in how the identity of the dopant affected the oxygen reduction reaction and how changes in the 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 oxygen. Other factors that may also contribute include bandgap states associated with the dopants and the fact that not all the dopants lead to the creation of Fe(II) sites.

See our paper:

B.A. Balko and K.M. Clarkson ('01), Journal of the Electrochemical Society, 148(2) E85-E91 (2001)

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