4.3 Stability measurements 71
approaching stable performance. However, at the same time a constant mass
loss takes place at a rate consistent with the potential range being between 1.8
and 1.9 VRHE.
Figure 4.6: Measured potential and mass loss during a two hour chronopotentiometry
test at 20 mA/cm2 in 1 M KOH. The left side axis (data in black) shows the
potential scale, corrected for Ohmic losses. The right side axis (data in blue) shows
the mass losses measured with the EQCM method.
Before conducting extrapolation based on the EQCM data, it is useful to con-
rm which elements actually dissolved into the electrolyte. While the EQCM
technique is prolic in giving time resolved information about the mass change,
it is also blind to what causes these changes. With Inductively Coupled Plasma -
Mass Spectrometry, ICP-MS, it is instead possible to analyse the concentrations
of elements in the electrolyte. With the combination of EQCM and ICP-MS we
can therefore compare the mass that left the electrode with the Mn concentration
increase in the electrolyte. The pure ICP-MS results are in the form of counts
registered with the mass spectrometer. The number of counts can be converted
into concentration values, CICP��MS (in mass per liter), using calibration with
standard solutions. However, to facilitate the comparison between EQCM and
ICP-MS the concentration values are further converted to ngMnO2/cm2, with
the following equation;
M = CICP��MS Velectrolyte
MMnO2
A MMn
(4.1)
where A is the active electrode area, MMnO2 the molar mass of MnO2, MMn
the molar mass of Mn and Velectrolyte the volume of the electrolyte solution at
the time the sample was taken. In gure 4.7 a comparison between results from
EQCM and ICP-MS can be seen. There are only minor discrepancies between