Environmental Engineering Reference
In-Depth Information
In water-splitting studies the applied potential is reported against RHE, thus the poten-
tial measured with the Ag/AgCl electrode must be converted into RHE scale using the
following expression:
E Ag / AgCl vs . SHE +
E RHE
=
E Ag / AgCl +
0 . 059
×
pH
(10.2.14)
where the E Ag / AgCl vs . SHE is the potential of the Ag/AgCl reference electrode with respect
to the SHE, see Table 10.2.3 (Peterson, 2012).
There are experimental reasons for the choice of a reference electrode; one impor-
tant selection parameter is its stability on the electrolyte solution where it is immersed
as well as the operating temperature (Peterson, 2012). All the reference electrodes
are very sensitive and so it is crucial their good maintenance; the lifetime of a ref-
erence electrode is 2-3 years with a daily basis usage. The feasibility of a reference
electrode can be checked using three identical reference electrodes and by observing
potential differences at every two weeks and confirming if the deviation between any
two individual electrodes is less than
±
3 mV (Krol and Grätzel, 2012).
10.3 ELECTROCHEMICAL IMPENDANCE SPECTROSCOPY
Several photoelectrochemical techniques have been used to characterize photoelec-
trodes with the goal of understanding its performance and limitations. This kind
of measurement is usually performed under steady-state and includes simulated sun-
light measurements, as photocurrent-voltage, wavelength-dependent measurements,
photocurrent action spectra and quantum efficiencies. Nevertheless, more detailed
properties cannot be extracted from steady-state measurements and so dynamic tech-
niques should be considered to identify performance-limiting steps or to determine
certain materials properties. These techniques allow the interpretation of the charge
transfer kinetics, mainly characterized by diffusion coefficients and lifetime of the
different charge carriers. One of the most powerful characterization techniques of
photoelectrochemical cells involving transient probing is Electrochemical Impedance
Spectroscopy (EIS).
EIS is a dynamic technique that has many advantages, not only because it is user-
friendly, but also because of to its sensitivity and ability to separate different complex
processes, such as those occurring in a photoelectrochemical system (Bisquert, 2002;
Bisquert, 2003). The foundations of EIS began in the 19th century with the con-
troversial but extraordinary work of Oliver Heaviside (Heaviside, 2012), where he
defined the terms “impedance'', “reactance'' and “admittance''. At the end of the 19th
century, Warburg derived the impedance function for a diffusional process in a remark-
able work where he extended the concept of impedance to electrochemical systems
(Macdonald, 2006). In the early 20th century, EIS experiments were performed
mainly for capacitance measurements of ideally polarizable electrodes, e.g. mercury,
using reactive bridges at relatively high frequencies. Even though these studies yielded
important information about the double layer behavior, complete impedance studies
including the low frequency range were only possible at the 1940s with the invention
of electronic potentiostats. Three decades later, the frequency response analyzer (FRA)
was developed, allowing one to probe electrochemical interfaces at sub-millihertz range
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