Standard Hydrogen Electrode consists of platinum wire fused in a glass tube and a platinum plate coated with finely divided platinum black. which also act as the electrode. why is the platinum coated with finely divided platinum black?

To increase surface area of the platinum electrode which results in superior quality and action of the electrodes as opposed to normal platinum electrodes.

Explanation:

Platinization of Platinum is the process of covering platinum electrode with a layer of platinum black. Platinum black is a finally divided form of platinum, optimized for catalysing the addition of hydrogen to unsaturated organic compound. This increases the surface area of the platinum electrodes and therefore exhibits action superior to that of normal electrodes.