Photoelectrochemical cells for hydrogen production from solar energy - Solar Energy Sciences and Engineering Applications - page 310

Table 10.2.3 Common reference electrodes for PEC research overview (Peterson, 2012).

Reference Electrode Filling Solution Potential (vs. SHE)

Reversible Hydrogen The electrode is in the actual E 0 = 0.0 + 0.059 × pH

Electrode (RHE) electrolyte solution and not

separated by a salt bridge

Standard Hydrogen Acid solution with activity E 0 = 0.0

Electrode (SHE) equal to 1

( = normal hydrogen) [H + ] = 1.18M

Calomel (Hg/Hg 2 Cl 2 ) 0.1M KCl 0.334

1M KCl ( NCE ) 0.281

3.5M KCl 0.250

Saturated KCl ( SCE ) 0.242

Saturated NaCl ( SSCE ) 0.236

Silver/Silver chloride (Ag/AgCl) 0.1M KCl 0.288

1M KCl 0.237

3M KCl 0.210

3.5M KCl 0.205

Saturated KCl 0.198

3M NaCl 0.209

Saturated NaCl 0.197

SeaWater 0.25

counter-electrode. The reaction at the counter-electrode should be as fast as possible
and should have a high catalytic activity in order to prevent performance limitations
(Krol and Grätzel, 2012). Usually, platinum is used as counter-electrode; this material
presents good stability over a wide range of electrolytes and pH, as well as showing
low overpotentials for hydrogen evolution (

0.1 V). To avoid inhomogeneous current
densities at the working electrode, the counter-electrode should face it symmetrically;
this is critical for electrolytes concentration lower than 0.5 M (Krol and Grätzel, 2012).
Moreover, the counter-electrode area should be twice as large than the photoelectrode
area (Krol and Grätzel, 2012). In PEC systems, a compromise must be maintained
between the working electrode, the counter electrode and the electrolyte solution in
order to ensure low overpotentials, fast charge transport and efficient light absorption.

∼

10.2.5.3 The reference-electrode
In single photon-system PEC cells, the applied potential is an important parameter
when studying the properties of the photoelectrode (photoanode or photocathode).
The three-electrode configuration allows one to measure the applied potential with
respect to a fixed reference electrode, allowing to turn visible the independent response
of the working electrode to any change in the applied potential. The same cannot be
hold for the metal counter-electrode since its overpotential at the interface with the
electrolyte is usually unknown and varies with the amount of current flowing through
the cell according to the Butler Volmer relation. Table 10.2.3 shows an overview
considering the reference electrodes most used in PEC research applications.
As exemplified in Table 10.2.3 there are several choices for the reference electrode.
However, the most commonly used reference electrode is the silver/silver chloride.