3.3 Electrochemical measurements 57
Figure 3.10: Image showing the three electrode setup in a rotating disk electrode
station. The most important components are highlighted with a text label, including
counter electrode (carbon rod), working electrode (attached to rotator) and reference
electrode (here a Hg/HgSO4 electrode). Figure taken from 186.
perform a cathodic reaction, normally hydrogen evolution. However, the potential
dierence between the counter and working electrode is a function of the
catalytic activities of both, which makes it complicated to study one catalyst at
a time. To this end, the potential applied on the working electrode is measured
against a known reference electrode, composed of a redox couple in equilibrium.
The potential of this reference can then be measured against a hydrogen electrode
and the results can be reported using the Standard Hydrogen Electrode,
SHE, or Reversible Hydrogen Electrode, RHE, scale. The standard hydrogen
electrode scale is based on the consensus of a zero electrochemical potential for
the hydrogen evolution reaction in equilibrium at standard conditions on a Pt
electrode. For RHE the eect of pH is also taken into account, meaning that
59 mV per pH increase has to be added. Using a reference electrode makes the
measurement of the working electrode independent of the catalytic activity of
the counter electrode, which can then be selected more freely. Still, the counter
electrode must provide sucient surface area so that the overall reaction does
not become limited by the number of sites available for the reduction reaction.
Throughout this project a high surface area graphite rod has therefore been
used.