standard hydrogen electrode

A standard hydrogen electrode (SHE) is the cathode of a galvanic cell where the reduction of hydrogen is carried out:

$2\mathrm{H}{}^{+}(\mathrm{aq})+2\mathrm{e}{}^{-}\to \mathrm{H}{}_2(\mathrm{g})$

When $[\mathrm{H}{}^{+}]=1\mathrm{M}$ or when $\mathrm{pH}=0$, $P=1\mathrm{atm}$, and a $\mathrm{Pt}$ electrode is used, ${\xi }^{\circ }=0\mathrm{V}$. This allows us to set the standard reduction potentials of other half-reactions. Note that when $\mathrm{pH}=7$, SHE will have a non-zero voltage of $0.42\mathrm{V}$.

Electrode material must be inert

Since $\mathrm{H}{}^{+}$ is acidic, the electrode must made of platinum or gold (inert to acidification / $\mathrm{H}{}^{+}$). Gold specifically needs two acids to dissolve. Normally platinum is used.