Which precious metal is used in the standard hydrogen electrode?

The precious metal used in the standard hydrogen electrode is platinum.

A standard hydrogen electrode, or SHE, uses a platinum electrode in contact with hydrogen gas and an aqueous solution containing hydrogen ions. Platinum provides an electrically conductive, chemically resistant surface where hydrogen can be oxidized or hydrogen ions can be reduced. It participates as a catalyst and conductor rather than being consumed in the overall reaction.

Under standard conditions, hydrogen gas is assigned an activity of 1, hydrogen-ion activity is 1, and the electrode’s potential is defined as exactly 0 volts. Other electrode potentials are measured relative to this reference, making the SHE the foundation of the standard reduction-potential scale.

Platinum is especially suitable because it is relatively unreactive and can support finely divided platinum black, which increases surface area and helps the hydrogen reaction proceed. The electrode is not simply a platinum wire dipped into water: the hydrogen pressure, solution composition, cleanliness, and gas–liquid interface all affect its behavior. Silver, gold, and nickel may serve in other electrochemical electrodes, but not as the canonical SHE electrode material.