6.4 Acid leaching 109
sample as a consequence of further electrochemical cycling in 10 Torr of O2. This
time the cathode was subjected to a total of 115 potential cycles between -0.4 V
and 1.4 V. After this treatment the percentage of metallic Y decreased further from
27 % to only 20 %.
6.4 Acid leaching
After various combinations of electrochemical cycles under different H2O and O2
pressures, the membrane was extracted from the high-pressure cell. The cathode
surface was exposed to a drop of 0.1 M H2SO4 electrolyte for 10 minutes followed
by various drops of water for rinsing. This procedure was repeated two times
in order to ensure a good acid leaching and the removal of the oxidized Y. The
new measurements acquired in vacuum exhibited in average much lower levels of
oxidized Y as apparent from Figure 6.6, where the metallic component represents
54 % of the total Y signal. It was still impossible to achieve a complete oxide
removal with this procedure.
After frontside: vacuum
Y metallic (54%)
Normalized intensity (a. u.)
Binding energy / eV
Figure 6.6: Typical Y 3d XPS spectrum after ex-situ acid leaching in 0.1 M H2SO4 electrolyte.
The metallic component of Y increased to 54 % of the total Y area.
Figure 6.7 a shows a broad XPS spectrum including both the Pt 4f and Y 3d
peaks and how their areas were calculated for estimating the Pt to Y ratios. The
evolution of the Pt to Y XPS ratios is plotted in Figure 6.7 b for an average of at least
three independent measurements in different spots for the membrane as-prepared,
after EC mesurements and after ex-situ acid leaching. The ratios are expressed both
in terms of Pt to total Y signal and in terms of Pt to metallic Y. In both cases the
ratios are quite reproducible before electrochemical measurements confirming the