Introduction

Introduction#

When you do an electrochemical experiment, the composition of your electrode can change depending on the conditions. For example, a metal electrode can oxidize, forming a metal oxide. Under different conditions, the metal can dissolve. The two main parameters affecting stability in a water-based electrolyte are electrode potential and pH. A Pourbaix diagram visualizes which material phase is stable under which conditions.

../../_images/iron-pourbaix.png

Fig. 1 Pourbaix diagram of iron (Wikipedia).#

Water stability#

The dashed blue lines in the Pourbaix diagram above indicate the water stability window. If you go below the lower line, you produce hydrogen according to the reaction

(1)#\[ \mathrm{2H^+ + 2e^- \to H_2(g)}. \]

This reaction is called the hydrogen evolution reaction (HER). Above the upper line, the oxygen evolution reaction (OER)

(2)#\[ \mathrm{2H_2O \to O_2(g) + 4H^+ + 4e^-} \]

occurs. Typically, an experiment is done within the water stability window to avoid the formation of gas bubbles.

Questions

  • What iron oxide phases can you encounter in a typical electrochemical experiment?

  • What do the HER and OER look like in alkaline media?

Reference points and SHE#

Throughout this lab course, the concept of a ‘reference point’ will return several times. We cannot measure an absolute value for electric potentials and energies; we always measure the difference with respect to some reference point.

In electrochemistry, the most common reference point for the electrode potential is the standard hydrogen electrode (SHE). It’s defined as a platinum electrode where the hydrogen evolution reaction (1) occurs under standard conditions (1 bar \(\mathrm{H_2}\)-pressure, 1 M \(\mathrm{H^+}\)-concentration, i.e. pH 0). When using the SHE as a reference, the equilibrium potential of the SHE is defined as zero.

Questions

  • What point in the Pourbaix diagram corresponds to the SHE?

In experiments, we often use a different reference electrode: the reversible hydrogen electrode (RHE). It’s almost the same as SHE, except that the \(\mathrm{H^+}\)-concentration is allowed to change, depending on your experiment. The zero on the RHE scale corresponds to the blue dotted line in the Pourbaix diagram Pourbaix diagram of iron (Wikipedia)..

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