60 Experimental Methods
of electrochemically active metal atoms at the surface. Such an estimation is
valuable due to discrepancies between geometric surface area and electrochemically
accessible surface area. Another way to estimate the surface area is using
the capacitive response, which is a function of the scan rate. The capacitative
response is based on charging of the electrode electrolyte interface when the potential
changes and is considered proportional to the number of active surface
species 109. However, for oxides there is a lack of consensus and benchmarks
regarding normalisation factors and therefore attempts to normalise currents
with electrochemical surface area are rarely reported.
3.4 Stability measurements
3.4.1 Electrochemical Quartz Crystal Microbalance
In order to analyse stability for electrocatalysts, it is complicated to rely on
purely electrochemical current-potential measurements. In theory, a corrosion
or dissolution process could occur through a well dened oxidation or reduction
process that could be analysed using the amount of charged passed. However,
catalyst stability is mostly relevant under conditions where a catalytic reaction
takes place, in this project oxygen evolution. Therefore the dominant part of
the current can usually be ascribed to this reaction. Instead, the electrochemical
quartz crystal microbalance, EQCM, provides a way to monitor the mass
changes that occur during an electrochemical reaction 200,201. The technique
relies on measuring the resonant frequency of a quartz crystal which, for the
right crystal orientation, changes with the amount of material present on top.
An image and schematic of the QCM crystals used in this project can be seen
in gure 3.13.
Figure 3.13: Left: Image showing the two sides of EQCM crystals. Right: A cross
section schematic of the crytals is shown, indicating the layered structure that makes
it possible to measure electrochemistry and resonance frequency simultaneously.
The resonance in quartz arises from the piezoelectric eect, where mechanical