30 Electrocatalysis and the splitting of water
In acidic electrolyte, relevant for PEM cells, most experimental studies are
focused on understanding and improving the noble metal oxides, RuOx and
IrOx. In a study by Reier et al. Ru, Ir and Pt electrodes were studied for
activity towards OER 100. They paid special attention to the electrochemical
active surface area which was determined by CO adsorption prior to activity
measurement. Here it should be noted that addressing the electrochemical surface
area for oxides is not well established due to a lack of accurate methods 74.
However, for metallic surfaces it can be done with high precision; to a rst degree
the electrochemically formed oxide area should not vary much from the
initial metallic. On this basis Ru outperformed Ir and Pt. At the same time
it was also evident that Ru nanoparticles were too unstable under the reaction
conditions. This can be seen in gure 2.8 where the activity is normalised to
the amount of CO adsorbed.
Figure 2.8: Comparison of results for anodic polarisation of Ru, Ir and Pt polycrystalline
disks (bulk) and nanoparticles. Ru is shown in black, Ir in red and Pt in
blue. Bulk measurements are indicated with dashed lines while nanoparticles are in
solid lines. The current is normalised with the amount of CO adsorbed on the surface,
based on electrochemical CO stripping. Figure taken from 100.
Since the activity of RuOx based catalysts is high compared to other oxides, it
is interesting to investigate whether further activity enhancements are possible.
It has indeed been shown to be the case in various studies, where especially
mixtures with Ni or Co stand out 104107. A theoretical model explaining
these enhancements has recently been proposed on the basis of an extended
thermodynamical analysis 108. In this model the adsorption of OOH, which
was shown to be a limiting factor, can be stabilized by introducing a hydrogen
acceptor close to the catalytic site. If this acceptor can retrieve the hydrogen
atom from OOH (and regenerate) at potentials close to OER, the overpoten