40 Electrocatalysis and the splitting of water
IrOx dissolves primarily at potentiodynamic conditions it is likely that the mode
of operation will have a large impact on the lifetime, where less start-stops lead
to a longer lifetime.
The dissolution products can in some cases be detected electrochemically with
a separate electrode during a catalyst test. This is the case for ruthenium
dissolving as RuO4, which can be reduced at 1.1 VRHE 181, 182. In practice,
this type of test is carried out with a Rotating Ring Disk Electrode, RRDE,
setup where the catalyst is loaded as a standard rotating disk and the separate
electrode is positioned as a ring around the disk. This geometry allows for an
accurate determination of the eciency with which the ring picks up products
from a reaction at the disk. A few groups have characterized Ru dissolution with
this method and the Faradaic eciency for RuO4 formation is between 6 and 15
% 40,182,183. It should be noted that this technique is also frequently used for
the determination of Faradaic eciency towards oxygen evolution 101,109,139.
A combination of RRDE and ICP-MS measurements were used by Markovic
and co-workers 183 to elucidate trends of stability among the metals that can
catalyse OER, see gure 2.14a. They found that the more unstable metals were
also more active catalysts. In two other studies from the same group, focused
on mixed Ru-Ir thin lms 184 and SrRuO3 185, this argument is further
supported. However, the trend found by Cherevko et al., which can be seen in
gure 2.14b, appears to be in contrast to the simple inverse stability-activity
relationship.
OsRuIrPtAu
1,0
0,8
0,6
0,4
0,2
0,0
/V
20
15
10
5
0
Noblemetalions/ppb
RuIrRhPdPtAu
1,0
0,8
0,6
0,4
0,2
0,0
/V
2,4
2,1
1,8
0,06
0,03
0,00
a)
Dissolutionrate/ng.cm-2.s-1
b)
Figure 2.14: a) Stability and activity trends for Os, Ru, Ir, Pt and Au under a
constant current density of 5 mA/cm2. The stability is measured as concentration of
metals in solution after test (ppb). The activity is shown as the overpotential needed
to reach 5 mA/cm��2. Data from 183. b) Trends of activity and stability for Ru, Ir,
Rh, Pd, Pt and Au under a constant current density of 0.8 mA/cm2. The dissolution
rate, in ng.cm��2.s��1 is found with online ICP-MS at a constant current density of 0.8
mA/cm��2. Note the scale break in the dissolution rate. The activity is shown as the
overpotential needed to reach 0.8 mA/cm��2. Data from 179.
From the two gures 2.14a and b, the activity trends are the same, but in 2.14b