2.5 Oxygen Evolution 29
Figure 2.7: Volcano shaped activity plot for oxygen evolution on various oxide
surfaces. The activity is shown as overpotential needed for all reaction steps to be
downhill in energy, which is then plotted as function of the dierence in free energy
for step 1 and 3, G0
O �� G0
HO . The points are calculated values for the indicated
surfaces, while the solid lines are derived from scaling relations between the OER
intermediates. The gure is from 98.
2.5.2 Recent experimental advances
With theory and experiments in good agreement, it is evident that RuO2 is a
unique catalyst for OER, which has been shown in several studies 40, 99101.
Unfortunately, as it has been mentioned earlier, ruthenium is a scarce element
produced as a by-product in platinum extraction. It is therefore problematic
to use for technologies that are to be scaled up to the terawatt level 102. At
the same time the stability of RuOx based electrodes have turned out to be
unsatisfactory for long lifetime of electrolyzers and many commercial anodes for
electrolysis instead use iridium or mixtures of the two 41, 103. Iridium is even
more scarce and therefore a search for alternative catalysts has long been the
focus in the research eld. The experimental progress can be divided into eorts
aimed at acidic electrolytes and eorts aimed at alkaline electrolytes.